John Brennan is shown speaking here at the Richard Nixon Presidential Library in Yorba Linda, Calif., in 2016, when he was serving as CIA director. Brennan says he is considering a legal challenge to President Trump's decision to revoke his security clearance. Chris Carlson/AP hide caption
Former CIA Director John Brennan in 2017. A group of top former officials who served under both Republican and Democratic administrations called the revocation of Brennan's security clearance "unprecedented." Joe Raedle/Getty Images hide caption
Former CIA Director John Brennan arrives at a May 2018 Senate hearing. President Trump revoked Brennan's security clearance Wednesday, claiming Brennan was acting "erratic." Alex Wong/Getty Images hide caption
Former CIA Director John Brennan at a congressional hearing in 2017. President Trump revoked Brennan's security clearance Wednesday, claiming he was "erratic." Brennan has been one of Trump's harshest critics in the national security community. Alex Wong/Getty Images hide caption
Former CIA Director John Brennan and other former top national security officials could lose their security clearance if the White House follows through on a threat made Monday. Claire Harbage/NPR hide caption
Since stepping down as CIA chief last year, John Brennan has been a harsh critic of President Trump. "I just think he hasn't fulfilled the responsibilities of the president of the United States," Brennan tells NPR. Claire Harbage/NPR hide caption
From left, National Security Agency Director Adm. Mike Rogers, former FBI Director James Comey, former Director of National Intelligence James Clapper, ex-CIA Director John Brennan, and Defense Intelligence Agency Director Lt. Gen. Vincent Stewart participate in the Senate intelligence committee's hearing on worldwide threats last year. Alex Brandon/AP hide caption
CIA Director John Brennan at CIA headquarters on Thursday. Ariel Zambelich/NPR hide caption
CIA Director John Brennan. Andrew Harnik/AP hide caption
CIA Director John Brennan discussed ISIS, the FBI-Apple dispute over an iPhone, the state of the Iran nuclear deal, and his future plans as President Obama's term draws to a close. Ariel Zambelich/NPR hide caption
CIA Director John Brennan listens during a news conference at CIA headquarters in Langley, Va., in December 2014. Pablo Martinez Monsivais/AP hide caption
CIA Director John Brennan Carolyn Kaster/AP hide caption
Senate Majority Leader Harry Reid faces reporters on Capitol Hill in Washington on March 11, following a caucus lunch. J. Scott Applewhite/AP hide caption
Sen. Dianne Feinstein speaks to reporters after speaking about her oversight committee's problematic relationship with the CIA Tuesday. CIA Director John Brennan says his agency isn't trying to delay the panel's report on the U.S. interrogation program. Mark Wilson/Getty Images hide caption
The seal of the CIA at the agency's headquarters in Virginia. Greg E. Mathieson Sr./MAI/Landov hide caption
Sen. Rand Paul, R-Ky., during his filibuster. Senate Television/AP hide caption
John Brennan, President Obama's nominee to head the CIA, prepares to testify at his confirmation hearing before the Senate Intelligence Committee on Thursday. J. Scott Applewhite/AP hide caption
John Brennan, President Obama's nominee to be the next CIA director, worked closely with Saudi Arabia to set up a secret U.S. drone base there, The New York Times reported. Brennan's confirmation hearing is Thursday. Kevin Dietsch/UPI /Landov hide caption
John Brennan, President Obama's nominee to be the next CIA director. Kevin Dietsch/UPI /Landov hide caption
The nominees: John Brennan (left), who is President Obama's choice to head the CIA; and former Sen. Chuck Hagel, R-Neb., who the president has asked to be secretary of defense. STAFF/Reuters /Landov hide caption
John Brennan, who President Obama wants to lead the CIA. Alex Wong/Getty Images hide caption