"Let me ask you: Have you even read the United States Constitution?" Khan said directly to Donald Trump from the DNC podium. "I will gladly lend you my copy."
Gold Star father Khizr Khan
Gold Star father Khizr Khan
Gold Star father Khizr Khan, whose son Army Captain Humayun Khan was killed in Iraq saving fellow soldiers, campaigns for Hillary Clinton, Tim Kaine, and LuAnn Bennett at the Leesburg Coordinated Campaign Office on November 4, 2016 in Leesburg, Virginia.
(Photo by Leigh Vogel/WireImage)
Khizr Khan, whose son, Humayun S. M. Khan was one of 14 American Muslims who died serving in the U.S. Army in the 10 years after the 9/11 attacks, offers to loan his copy of the Constitution to Republican U.S. presidential nominee Donald Trump, as he speaks while a relative looks on during the last night of the Democratic National Convention in Philadelphia, Pennsylvania, U.S. July 28, 2016.
Gold Star father Khizr Khan speaks before introducing U.S. Democratic presidential nominee Hillary Clinton at a campaign rally in Manchester, New Hampshire, U.S., November 6, 2016.
Gold-star father Khizr Khan, father of U.S. Army Captain Humayun Khan who was killed in 2004 in Iraq, takes part in a discussion panel on the Muslim and Refugee ban in the U.S. Capitol in Washington, U.S., February 2, 2017.
Gold-Star father Khizr Khan, the father of U.S. Army Captain Humayun Khan who was killed in 2004 during the Iraq War, speaks during a news conference held by House Democrats to introduce legislation "to ensure that no one is denied entry into the United States because of their religion" at the U.S. Capitol in Washington February 2, 2017.
Khizr Khan, and his wife Ghazala (L), whose son, Humayun S. M. Khan was one of 14 American Muslims who died serving in the U.S. Army in the ten years after the 9/11 attacks, appear with Democratic U.S. presidential nominee Hillary Clinton supporters at her election night rally in New York, U.S., November 9, 2016.
Gold-Star father Khizr Khan, father of U.S. Army Captain Humayun Khan who was killed in 2004 in Iraq, puts his hand to his heart as he takes part in a discussion panel on the Muslim and refugee ban in the U.S. Capitol in Washington February 2, 2017.
Council on American-Islamic Relations National Executive Director Nihad Awad, U.S. Rep. Keith Ellison (D-MN), and Gold Star father Khizr Khan listen during a news conference in front of the Capitol February 1, 2017 on Capitol Hill in Washington, DC. Rep. Keith Ellison (D-MN) hosted the press conference to discuss President Donald Trump's travel ban, which prevents immigrants and refugees from seven Muslim-majority countries from entering the U.S, and objections to Senator Jeff Sessions' nomination to the position of Attorney General.
(Photo by Alex Wong/Getty Images)
BEVERLY HILLS, CA - NOVEMBER 13: Khizr Khan attends the ACLU SoCal's 2016 Bill of Rights Dinner at the Beverly Wilshire Four Seasons Hotel on November 13, 2016 in Beverly Hills, California. (Photo by Amanda Edwards/WireImage)
Khizr Khan attends the ACLU SoCal's 2016 Bill of Rights Dinner at the Beverly Wilshire Four Seasons Hotel on November 13, 2016 in Beverly Hills, California.
(Photo by Amanda Edwards/WireImage)
Khizr Khan, a Virginia Gold Star father, whose son Army Captain Humayun Khan was killed in Iraq, campaigns for Hillary Clinton at Croaker's Spot, a restaurant in Norfolk, Va. on October 26, 2016.
(Photo by Julia Rendleman for The Washington Post via Getty Images)
Khizr Khan gestures after speaking in support of Democratic presidential nominee Hillary Clinton during election night outside the Jacob K. Javits Convention Center in New York on November 8, 2016.
(KENA BETANCUR/AFP/Getty Images)
Khizr Khan speaks at Democratic presidential nominee Hillary Clinton's election night party at Javits Center on November 8, 2016 in New York City.
(Photo by Noam Galai/FilmMagic)
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Khan made a lasting impact when he spoke about his son, Humayun Khan, who was killed in action in 2004 while on tour in Iraq. Khan used his platform at the DNC to speak directly to Trump, who was then the presumptive Republican nominee for president, asking what he had sacrificed for the country.
He also questioned Trump's opinion of Muslims and other minority groups. Khan made several appearances on television news shows and political ads, cementing himself as an outspoken critic of Trump, his policies and his administration.
On Monday, he was on his way to a speaking event in Canada — until he reached out to the event organizer, claiming his freedom to travel in and out of the U.S. was not guaranteed.
Also on Monday, President Trump privately signed a more narrowly written executive order to replace the travel ban currently tied up in the courts. This new order highlights six Muslim-majority countries barred from open travel to the U.S. It doesn't list Iraq, which was included in the original travel ban.
Khan has since been unreachable or unwilling to give more information about the situation. And that's led to questions about the truthfulness of his story.
Khan was born in Pakistan but has been a U.S. citizen for more than three decades.