Mick Mulvaney, Trump's former acting chief of staff, testifying before Jan. 6 committee

WASHINGTON — Mick Mulvaney, who served as acting chief of staff under former President Donald Trump, will sit for a virtual taped deposition with the House committee investigating Jan. 6 on Thursday, three sources familiar with the plans told NBC News.

Mulvaney, Trump's acting chief of staff in 2019 and early 2020, arrived for his closed-door deposition at around 1:40 p.m. Asked by NBC News what he would tell the committee, he said, “The truth. How about that for a start."

When Trump tapped Mark Meadows to replace Mulvaney as chief of staff, he appointed the former South Carolina congressman, who also served as director of the Office of Management and Budget, as the special envoy to Northern Ireland.

Mulvaney told CNBC he resigned from that role on the night of the Capitol riot. "I called [then-Secretary of State] Mike Pompeo last night to let him know I was resigning from that. I can’t do it. I can’t stay,” he said on Jan. 7 in an interview on "Squawkbox."

“Those who choose to stay, and I have talked with some of them, are choosing to stay because they’re worried the president might put someone worse in."

CBS News, which hired Mulvaney as contributor in March, was first to report on his scheduled testimony before the committee.

Pompeo said Thursday that he's had discussions about potentially speaking with the committee.

“We’ve had discussions with them about, uh, potentially appearing before them, trying to make sure we understand what it is, uh, they are asking for," he said in an interview on Fox News. "I am happy to cooperate with things that are fair, and transparent, and deliver good outcomes to the American people.”

The House Jan. 6 committee has been airing public hearings about the lead-up to that day and the actions Trump took, or did not take, as the insurrection unfolded. Rep. Liz Cheney, R-Wyo., vice chair of the panel, has said the investigation is ongoing, new witnesses have come forward and they will hold more hearings in September.

The Department of Justice is also investigating Trump's actions leading up to the riot as part of its criminal probe of efforts to overturn the 2020 election results, an administration official familiar with the investigation said this week.

The spokesperson for the committee told NBC News Thursday that it has created a formal path to share investigative material, including witness transcripts and evidence, with the Justice Department, confirming remarks by its chair Rep. Bennie Thompson, D-Miss., to Politico.

“We’ve put a template together for sharing information, sharing it with Justice. My understanding is, there’s general agreement on it,” Thompson told the outlet.

Mulvaney tweeted last month that he believes Cassidy Hutchinson, the former top aide to Meadows, who testified that Trump got into a physical altercation with a Secret Service officer in an attempt to join his supporters at the Capitol on the day of the riot.

"My guess is that before this is over, we will be hearing testimony from [Secret Service official Tony] Ornato, [then-head of Trump's security detail Bobby] Engle, and Meadows," he wrote. "This is explosive stuff. If Cassidy is making this up, they will need to say that. If she isn’t they will have to corroborate. I know her. I don’t think she is lying."

After members of the panel revealed last month at a public hearing they were worried that Trump allies were trying to intimidate witnesses cooperating with their investigation, Mulvaney said that could represent a "serious problem" for the former president.

"The Press is going to focus on some sensational revelations from today: guns, grabbing a secret service agent, etc. But the real bomb that got dropped was the implied charge of witness tampering," he tweeted. "If there is hard evidence, that is a serious problem for the former President."