18 crucial days: What did the president know and when did he know it?

WASHINGTON — Special Counsel Robert Mueller is trying to piece together what transpired inside the White House over a critical 18-day period that began when senior officials were told that National Security Advisor Michael Flynn was susceptible to blackmail by Russia, according to multiple people familiar with the matter.

The questions about what happened between Jan. 26 and Flynn's firing on Feb. 13 appear to relate to possible obstruction of justice by President Donald Trump, say two people familiar with Mueller's investigation into Russia's election meddling and potential collusion with the Trump campaign.

Multiple sources say that during interviews, Mueller's investigators have asked witnesses, including White House Counsel Don McGahn and others who have worked in the West Wing, to go through each day that Flynn remained as national security adviser and describe in detail what they knew was happening inside the White House as it related to Flynn.

Some of those interviewed by Mueller's team believe the goal is in part to determine if there was a deliberate effort by President Trump or top officials in the West Wing to cover up the information about Flynn that Sally Yates, then the acting attorney general, conveyed to McGahn on Jan. 26. In addition to Flynn, McGahn is also expected to be critical to federal investigators trying to piece together a timeline of those 18 days.

Neither McGahn's lawyer nor the White House responded to requests for comment. A spokesman for the Special Counsel's office declined to comment.

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Retired Army Lt. Gen. Michael Flynn
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Retired Army Lt. Gen. Michael Flynn
National security adviser General Michael Flynn delivers a statement daily briefing at the White House in Washington U.S., February 1, 2017. REUTERS/Carlos Barria

Retired Lt. Gen. Michael Flynn arrives at the Trump Tower for meetings with US President-elect Donald Trump, in New York on November 17, 2016.

(EDUARDO MUNOZ ALVAREZ/AFP/Getty Images)

Retired United States Army lieutenant general Michael T. Flynn introduces Republican Presidential nominee Donald J. Trump before he delivered a speech at The Union League of Philadelphia on September 7, 2016 in Philadelphia, Pennsylvania. Trump spoke about his plans to build up the military if elected. Recent national polls show the presidential race is tightening with two months until the election.

(Photo by Mark Makela/Getty Images)

Retired Lt. Gen. Michael Flynn, at podium, and Republican presidential candidate Donald Trump attend a campaign event with veterans at the Trump International Hotel on Pennsylvania Ave., NW, where Trump stated he believes President Obama was born in the United States, September 16, 2016.

(Photo By Tom Williams/CQ Roll Call)

WASHINGTON, DC - FEBRUARY 01: White House Press Secretary Sean Spicer (L) yields the briefing room podium to National Security Adviser Michael Flynn February 1, 2017 in Washington, DC. Flynn said the White House is 'officially putting Iran on notice' for a recent missile test and support for Houthi rebels in Yemen. (Photo by Win McNamee/Getty Images)

Army Lt. Gen. Michael Flynn, director of the Defense Intelligence Agency, prepares to testify at a Senate Armed Services Committee hearing in Dirksen Building titled 'Current and Future Worldwide Threats,' featuring testimony by he and James Clapper, Director of National Intelligence.

(Photo By Tom Williams/CQ Roll Call)

White House National Security Advisor Michael Flynn (C) arrives prior to a joint news conference between Canadian Prime Minister Justin Trudeau and U.S. President Donald Trump at the White House in Washington, U.S., February 13, 2017. REUTERS/Carlos Barria
U.S. National Security Advisor Michael Flynn boards Air Force One at West Palm Beach International airport in West Palm Beach, Florida U.S., February 12, 2017. REUTERS/Carlos Barria
National security adviser General Michael Flynn arrives to deliver a statement during the daily briefing at the White House in Washington U.S., February 1, 2017. Picture taken February 1, 2017. REUTERS/Carlos Barria
White House National Security Advisor Michael Flynn (L) arrives at Joint Base Andrews, Maryland, U.S., February 10, 2017. REUTERS/Carlos Barria
WASHINGTON, DC - FEBRUARY 10: (AFP OUT) White House National Security Advisor Michael Flynn (R) walks down the West Wing Colonnade following a bilateral meeting between U.S. President Donald Trump and Japanese Prime Minister Shinzo Abe February 10, 2017 in Washington, DC. Trump and Abe are expected to discuss many issues, including trade and security ties and will hold a joint press confrence later in the day. (Photo by Chip Somodevilla/Getty Images)
ATLANTA, GA - FEBRUARY 02: (L-R) SCAD Dramatic writing professor Chris Auer, Producer Sandra Leviton, Executive producer and writer Michael Flynn, Tv literary agent Jeff Greenberg and Literary manager and producer Kaila York speak on stage during the 'Inside the Writers Room' event on Day One of aTVfest 2017 presented by SCAD on February 2, 2017 in Atlanta, Georgia. (Photo by Vivien Killilea/Getty Images for SCAD)
Lieutenant General Michael Flynn (ret.), National Security Advisor Designate speaks during a conference on the transition of the US Presidency from Barack Obama to Donald Trump at the US Institute Of Peace in Washington DC, January 10, 2017. / AFP / CHRIS KLEPONIS (Photo credit should read CHRIS KLEPONIS/AFP/Getty Images)
Retired Lieutenant General Michael Flynn, U.S. national security advisor, attends a swearing in ceremony of White House senior staff in the East Room of the White House in Washington, D.C., U.S., on Sunday, Jan. 22, 2017. Trump today mocked protesters who gathered for large demonstrations across the U.S. and the world on Saturday to signal discontent with his leadership, but later offered a more conciliatory tone, saying he recognized such marches as a hallmark of our democracy. Photographer: Andrew Harrer/Bloomberg via Getty Images
WASHINGTON, DC - JANUARY 20: Michael Flynn walks out after a morning worship service on Inauguration day at St. John's Episcopal Church in Washington, DC on Friday, Jan. 20, 2017. (Photo by Jabin Botsford/The Washington Post via Getty Images)
WASHINGTON, DC - JANUARY 20: Attorney General nominee Sen. Jeff Sessions (R-AL) (R), talks with National Security Advisor Michael Flynn inside of the inaugural parade reviewing stand in front of the White House on January 20, 2017 in Washington, DC. Donald Trump was sworn in as the nation's 45th president today. (Photo by Mark Wilson/Getty Images)
Retired Lieutenant General Michael Flynn, White House national security adviser-designate, center, stands in an elevator at Trump Tower in New York, U.S., on Thursday, Jan. 12, 2017. Donald Trump is slated to meet with AT&T Inc.'s top executives on Thursday to discuss the company's proposed $85.4 billion bid for Time Warner Inc., according to people familiar with the matter. The president-elect has said he opposes the deal. Photographer: John Taggart/Bloomberg via Getty Images
WASHINGTON, DC - JANUARY 20: Donald Trump's National Security Adviser Gen. Michael T. Flynn (R) arrives on the West Front of the U.S. Capitol on January 20, 2017 in Washington, DC. In today's inauguration ceremony Donald J. Trump becomes the 45th president of the United States. (Photo by Drew Angerer/Getty Images)
Lieutenant General Michael Flynn (ret.) and National Security Advisor Designate and Ambassador Susan Rice, National Security Advisor during a ceremonial passing of authority while participating in a conference on the transition of the US Presidency from Barack Obama to Donald Trump at the US Institute Of Peace in Washington DC, January 10, 2017. / AFP / CHRIS KLEPONIS (Photo credit should read CHRIS KLEPONIS/AFP/Getty Images)
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When did Trump learn Flynn lied to the FBI?

The obstruction of justice question could hinge on when Trump knew about the content of Flynn's conversations with Russia's ambassador to the U.S. during the transition, which were at the crux of Yates's warning, and when the president learned Flynn had lied about those conversations to the FBI, according to two people familiar with the Mueller probe.

Flynn pleaded guilty earlier this month to lying to the FBI on Jan. 24, an interview that took place the day after he was sworn in as national security adviser.

Related: Mueller has enough evidence to bring charges in Flynn investigation

Yates has testified to Congress that she informed McGahn on Jan. 26 that Flynn had not been truthful in statements to senior members of the Trump team, including Vice President Mike Pence, when he said he did not discuss U.S. sanctions with Russia's ambassador, Sergey Kislyak. Yates said Flynn was susceptible to blackmail by the Russians because he had lied about the contents of a phone call with Kislyak.

Trump's legal team and senior White House aides are refusing to say when and how the president first learned that Flynn had lied to the FBI. Yates testified that in her Jan. 26 meeting with McGahn he asked her about the content of Flynn's FBI interview.

"Mr. McGahn asked me how he did and I declined to give him an answer to that," Yates testified in May. She told Congress that it would have been inappropriate for her to tell McGahn whether Flynn had been truthful.

Related: Mike Flynn's son is subject of federal Russia probe

That same day, Jan. 26, McGahn also briefed Trump and some of his senior advisers on his conversation with Yates, according to then-White House Press Secretary Sean Spicer.

"Immediately after the Department of Justice notified the White House counsel of the situation, the White House counsel briefed the president and a small group of senior advisers," Spicer told reporters on Feb. 14.

Two former federal prosecutors who spoke to NBC News on the condition of anonymity said most lawyers in McGahn's position would have immediately gone to Flynn and asked him whether he lied to the FBI.

President Trump told NBC's Lester Holt in an interview on May 11 that he didn't ask for Flynn's resignation after Yates's warning because once McGahn looked into it, he "came back to me and [it] did not sound like an emergency."

The conversation with Kislyak that Flynn misled Pence and other officials about took place on Dec. 29, the same day the Obama administration announced new sanctions against Russia in retaliation for Moscow's interference in the U.S. presidential election.

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Former FBI Director Robert Mueller
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Former FBI Director Robert Mueller
WASHINGTON, DC - OCTOBER 28: Former FBI director Robert Mueller attends the ceremonial swearing-in of FBI Director James Comey at the FBI Headquarters October 28, 2013 in Washington, DC. Comey was officially sworn in as director of FBI on September 4 to succeed Mueller who had served as director for 12 years. (Photo by Alex Wong/Getty Images)
US President Barack Obama applauds outgoing Federal Bureau of Investigations (FBI) director Robert Mueller (L) in the Rose Garden at the White House in Washington on June 21, 2013 as he nominates Jim Comey to be the next FBI director. Comey, a deputy attorney general under George W. Bush, would replace Mueller, who is stepping down from the agency he has led since the week before the September 11, 2001 attacks. AFP PHOTO/Nicholas KAMM (Photo credit should read NICHOLAS KAMM/AFP/Getty Images)
Outgoing FBI Director Robert Mueller applauds key staff members during a farewell ceremony held for him at the Justice Department in Washington, August 1, 2013. On Monday the U.S. Senate confirmed former Deputy Attorney General James Comey to replace Mueller, who has led the bureau since shortly before the September 11, 2001, attacks on the United States. REUTERS/Jonathan Ernst (UNITED STATES - Tags: POLITICS CRIME LAW HEADSHOT)
391489 03: U.S. President George W. Bush speaks during a conference as he stands with Justice Department veteran Robert Mueller, left, who he has nominated to head the FBI, and Attorney General John Ashcroft July 5, 2001 the Rose Garden at the White House in Washington, DC. (Photo by Mark Wilson/Getty Images)
Outgoing FBI Director Robert Mueller stands for the national anthem during a farewell ceremony for him at the Justice Department in Washington, August 1, 2013. On Monday the U.S. Senate confirmed former Deputy Attorney General James Comey to replace Mueller, who has led the bureau since shortly before the September 11, 2001, attacks on the United States. REUTERS/Jonathan Ernst (UNITED STATES - Tags: POLITICS CRIME LAW)
Outgoing FBI Director Robert Mueller (L) reacts to a standing ovation from the audience, Deputy U.S. Attorney General James Cole (C) and U.S. Attorney General Eric Holder (R) during Mueller's farewell ceremony at the Justice Department in Washington, August 1, 2013. On Monday the U.S. Senate confirmed former Deputy Attorney General James Comey to replace Mueller, who has led the bureau since shortly before the September 11, 2001, attacks on the United States. REUTERS/Jonathan Ernst (UNITED STATES - Tags: POLITICS CRIME LAW)
Outgoing FBI Director Robert Mueller gestures during his remarks at a farewell ceremony held for him at the Justice Department in Washington, August 1, 2013. On Monday the U.S. Senate confirmed former Deputy Attorney General James Comey to replace Mueller, who has led the bureau since shortly before the September 11, 2001, attacks on the United States. REUTERS/Jonathan Ernst (UNITED STATES - Tags: POLITICS CRIME LAW)
FILE PHOTO -- U.S. Attorney General John Ashcroft (R) and FBI Director Robert Mueller speak about possible terrorist threats against the United States, in Washington, May 26, 2004. REUTERS/Kevin Lamarque/File Photo
Outgoing FBI Director Robert Mueller reacts to applause from the audience during his farewell ceremony at the Justice Department in Washington, August 1, 2013. On Monday the U.S. Senate confirmed former Deputy Attorney General James Comey to replace Mueller, who has led the bureau since shortly before the September 11, 2001, attacks on the United States. REUTERS/Jonathan Ernst (UNITED STATES - Tags: POLITICS CRIME LAW)
UNITED STATES - JUNE 19: Chairman Pat Leahy, D-Vt., right, and FBI Director Robert Mueller make their way to a Senate Judiciary Committee hearing in Dirksen Building on oversight of the FBI. (Photo By Tom Williams/CQ Roll Call)
Outgoing FBI Director Robert Mueller (C) delivers remarks at a farewell ceremony for him at the Justice Department in Washington, August 1, 2013. On Monday the U.S. Senate confirmed former Deputy Attorney General James Comey to replace Mueller, who has led the bureau since shortly before the September 11, 2001, attacks on the United States. Also onstage with Mueller are Deputy U.S. Attorney General James Cole (FROM L), U.S. Attorney General Eric Holder, former CIA Director George Tenet and TSA Administrator John Pistole. REUTERS/Jonathan Ernst (UNITED STATES - Tags: POLITICS CRIME LAW)
WASHINGTON, DC - MAY 15: (L-R) Rep. Steny Hoyer (D-MD), U.S. Attorney General Eric Holder, Homeland Security Secretary Janet Napolitano, FBI Director Robert Mueller and Immigration and Customs Enforcement Director John Morton attend the National Peace Officers' Memorial Service at the U.S. Capitol May 15, 2013 in Washington, DC. Holder and other members of the Obama administration are being criticized over reports of the Internal Revenue Services' scrutiny of conservative organization's tax exemption requests and the subpoena of two months worth of Associated Press journalists' phone records. (Photo by Chip Somodevilla/Getty Images)
FBI Director Robert Mueller testifies before the House Judiciary Committee hearing on Federal Bureau of Investigation oversight on Capitol Hill in Washington June 13, 2013. Mueller said on Thursday that the U.S. government is doing everything it can to hold confessed leaker Edward Snowden accountable for splashing surveillance secrets across the pages of newspapers worldwide. REUTERS/Yuri Gripas (UNITED STATES - Tags: POLITICS CRIME LAW)
Ukrainian President Viktor Yanukovych (L) welcomes FBI Director Robert Mueller during their meeting in Kiev June 5, 2013. REUTERS/Efrem Lukatsky/Pool (UKRAINE - Tags: POLITICS)
FBI Director Robert Mueller (L) arrives for the Obama presidential inauguration on the West Front of the U.S. Capitol January 21, 2013 in Washington. President Barack Obama was re-elected for a second term as President of the United States. Woman at right is unidentified. REUTERS/Win McNamee-POOL (UNITED STATES)
WASHINGTON, : FBI Director Robert Mueller answers questions before Congress 17 October 2002 on Capitol Hill in Washington. Mueller was testifying before the House and Senate Select Intelligence committees' final open hearing investigating events leading up to the September 11, 2001. AFP Photos/Stephen JAFFE (Photo credit should read STEPHEN JAFFE/AFP/Getty Images)
(L-R) CIA Director Leon Panetta, Director of National Intelligence James Clapper and FBI Director Robert Mueller testify at a Senate Intelligence Committee hearing on Capitol Hill in Washington, February 16, 2011. REUTERS/Jason Reed (UNITED STATES - Tags: POLITICS)
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Federal Bureau of Investigation (FBI) Director Robert Mueller (L) stand during the National Anthem alongside Attorney General Eric Holder (R) and Deputy Attorney General James Cole (C) during a farewell ceremony in Mueller's honor at the Department of Justice on August 1, 2013. Mueller is retiring from the FBI after 12-years as Director. AFP PHOTO / Saul LOEB (Photo credit should read SAUL LOEB/AFP/Getty Images)
399994 01: Federal Bureau of Investigation (FBI) Director Robert Mueller greets American forces on the American military compound at Kandahar Airport January 23, 2002 in Kandahar, Afghanistan. Mueller had lunch with FBI officials and Haji Gulali, commander of the Kandahar region. (Photo by Mario Tama/Getty Images)
UNITED STATES - JUNE 19: FBI Director Robert Mueller, center, talks with Chairman Pat Leahy, D-Vt., right, and Sen. Orrin Hatch, R-Utah, talk before a Senate Judiciary Committee hearing in Dirksen Building on oversight of the FBI. (Photo By Tom Williams/CQ Roll Call)
UNITED STATES - JUNE 06: OVERSIGHT HEARING ON COUNTERTERRORISM--Sen. Richard J. Durbin, D-Ill., Senate Judiciary Chairman Patrick J. Leahy, D-Vt., FBI Director Robert S. Mueller III, and Sen. Charles E. Grassley, R-Iowa, before the hearing. (Photo by Scott J. Ferrell/Congressional Quarterly/Getty Images)
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On Jan. 12, the Washington Post reported that Flynn had spoken on the phone with Kislyak on Dec. 29. The timing of the call raised questions about whether the two had discussed sanctions. Three days after the Post report, Pence publicly said he had been assured by Flynn that sanctions were not discussed.

Other senior Trump officials, including Spicer, also said publicly during the transition that Flynn did not discuss sanctions with Kislyak. Spicer repeated that line from the White House podium on Jan. 23, saying he had asked Flynn about it again the night before.

Court documents from Flynn's plea deal show Flynn had discussed sanctions with Kislyak on Dec. 29 in coordination with Trump transition officials. NBC News reported that Flynn spoke with his incoming deputy K.T. McFarland to discuss what to say to Kislyak about the new U.S. sanctions in order to keep Russia from retaliating.

In the second week of February, Flynn again told senior White House officials he had not discussed sanctions with Kislyak. Fresh questions arose at that time because Washington Post reporters had multiple sources saying the two men had discussed sanctions. Under repeated questioning by the senior officials, Flynn shifted his story, according to White House officials familiar with the matter.

Pence said he first learned that Flynn had misled him when the Post story was published on Feb. 9. Four days later, Yates's warning to McGahn became public in another Post story.

Only then, on Feb. 13, did Trump fire Flynn, saying he did so because he had misled Pence.

Justice Department officials who spoke to NBC News on the condition of anonymity said they had expected the White House to fire Flynn on Jan. 26 upon learning that he had lied to the vice president.

Instead, Trump fired Yates on Jan. 30, citing her refusal to enforce his executive order banning people from seven Muslim-majority countries from travelling to the U.S. Before she left, however, she made available, at McGahn's request, evidence she had that Flynn had not been truthful about his conversations with Kislyak, according to her congressional testimony.

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President Trump holds a rally in Pensacola, Florida
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U.S. President Donald Trump speaks at a rally in Pensacola, Florida, U.S., December 8, 2017. REUTERS/Carlo Allegri
U.S. President Donald Trump gestures as he departs a rally in Pensacola, Florida, U.S., December 8, 2017. REUTERS/Carlo Allegri
U.S. President Donald Trump speaks at a rally in Pensacola, Florida, U.S., December 8, 2017. REUTERS/Carlo Allegri
A supporter dressed as Uncle Sam walks around before U.S. President Donald Trump addresses a rally in Pensacola, Florida, U.S., December 8, 2017. REUTERS/Carlo Allegri
U.S. President Donald Trump gestures as he arrives for a rally in Pensacola, Florida, U.S., December 8, 2017. REUTERS/Carlo Allegri
U.S. President Donald Trump arrives for a rally in Pensacola, Florida, U.S., December 8, 2017. REUTERS/Carlo Allegri
U.S. President Donald Trump speaks about protesters carrying signs at a rally in Pensacola, Florida, U.S., December 8, 2017. REUTERS/Carlo Allegri
U.S. President Donald Trump arrives for a rally in Pensacola, Florida, U.S., December 8, 2017. REUTERS/Carlo Allegri
U.S. President Donald Trump uses his famous "You're fired" line at a rally in Pensacola, Florida, U.S., December 8, 2017. REUTERS/Carlo Allegri
A supporter holds up a sign as U.S. President Donald Trump holds a rally in Pensacola, Florida U.S. December 8, 2017. REUTERS/Kevin Lamarque
U.S. President Donald Trump holds a rally in Pensacola, Florida, U.S. December 8, 2017. REUTERS/Kevin Lamarque
U.S. President Donald Trump holds a rally in Pensacola, Florida, U.S. December 8, 2017. REUTERS/Kevin Lamarque
Attendees listen as the U.S. National Anthem is played during a campaign rally with U.S. President Donald Trump in Pensacola, Florida, U.S., on Friday, Dec. 8, 2017. Trump�gave his most full-throated endorsement yet of Alabama Senate candidate�Roy Moore, casting aside calls for to shun the former judge whos been accused of sexual misconduct while seizing on reports that questioned the credibility of his accuser.�Photographer: Nicole Craine/Bloomberg via Getty Images
U.S. President Donald Trump applauds as he speaks during a rally in Pensacola, Florida, U.S., on Friday, Dec. 8, 2017. Trump�gave his most full-throated endorsement yet of Alabama Senate candidate�Roy Moore, casting aside calls for to shun the former judge whos been accused of sexual misconduct while seizing on reports that questioned the credibility of his accuser.�Photographer: Nicole Craine/Bloomberg via Getty Images
U.S. President Donald Trump speaks during a rally in Pensacola, Florida, U.S., on Friday, Dec. 8, 2017. Trump�gave his most full-throated endorsement yet of Alabama Senate candidate�Roy Moore, casting aside calls for to shun the former judge whos been accused of sexual misconduct while seizing on reports that questioned the credibility of his accuser.�Photographer: Nicole Craine/Bloomberg via Getty Images
An attendee holds a sign reading 'CNN. MSNBC. Fake News.' during a campaign rally with U.S. President Donald Trump in Pensacola, Florida, U.S., on Friday, Dec. 8, 2017. Trump�gave his most full-throated endorsement yet of Alabama Senate candidate�Roy Moore, casting aside calls for to shun the former judge whos been accused of sexual misconduct while seizing on reports that questioned the credibility of his accuser.�Photographer: Nicole Craine/Bloomberg via Getty Images
A supporter cheers as US President Donald Trump speaks during a rally at the Pensacola Bay Center on December 8, 2017 in Pensacola, Florida. / AFP PHOTO / Nicholas Kamm (Photo credit should read NICHOLAS KAMM/AFP/Getty Images)
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Mueller is trying to determine why Flynn remained in his post for 18 days after Trump learned of Yates' warning, according to two people familiar with the probe. He appears to be interested in whether Trump directed him to lie to senior officials, including Pence, or the FBI, and if so why, the sources said.

If Trump knew his national security adviser lied to the FBI in the early days of his administration it would raise serious questions about why Flynn was not fired until Feb. 13, and whether Trump was attempting to obstruct justice when FBI Director James Comey says the president pressured him to drop his investigation into Flynn. Trump fired Comey on May 9.

Trump denies pressuring Comey to drop the Flynn investigation, and his legal team has disputed any notion of the president obstructing justice.

Trump raised new questions about possible obstruction of justice on Dec. 2 when he wrote on Twitter that he fired Flynn because he had lied to Pence and the FBI, suggesting he already knew Flynn was in legal jeopardy for lying to federal investigators at the time he fired Comey.

"I had to fire General Flynn because he lied to the Vice President and the FBI," Trump wrote. "He has pled guilty to those lies. It is a shame because his actions during the transition were lawful. There was nothing to hide!"

Trump lawyer John Dowd later took responsibility for the tweet, saying he had drafted it for the White House social media director to post.

During her testimony before a Senate subcommittee in May, Yates testified that she didn't tell McGahn in their Jan. 26 meeting what Flynn told the FBI because that was under investigation. She said: "He asked me how he had done in the interview, and I specifically declined to answer that."

Based on Yates's testimony, McGahn's tone appears to have hardened in his second meeting with Yates the following day, Jan. 27, which took place after he spoke with Trump and other advisers. Yates testified that during that Jan. 27 meeting he questioned her about why it mattered if Flynn misled Pence. "Essentially: 'What's it to the Justice Department if one White House official is lying to another?'" she said McGahn asked her.

White House Press Secretary Sarah Sanders has repeatedly referred questions to Dowd about when Trump knew Flynn lied to the FBI. Dowd has declined multiple requests to answer that question.

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