Flynn, President Trump’s national security advisor, resigned late Monday night after it was revealed that he misled Vice President Mike Pence about his conversations with a top Russian diplomat. Flynn bowed out after days of mixed messages from the White House and nearly one month after former acting attorney general Sally Yates expressed concern about the nature of his communications, according to the Washington Post.

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The Post reports that Yates—along with former national intelligence director James Clapper Jr. and CIA director John Brennan—told the incoming administration that “Flynn had put himself in a compromising position” at the end of last year and was vulnerable to blackmail because of his potentially illegal discussions of U.S. sanctions with the diplomat. (At the time of his conversations with the Russian ambassador, he was not yet a member of the administration and so could be in violation of the Logan Act, which prohibits private citizens from interfering in diplomatic disputes.)

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Flynn had originally told Pence that his conversations with ambassador Sergey Kislyak were limited to small talk, information that the vice president later used to defend the security advisor in interviews with the press. Later, however, the FBI discovered that the contents of Flynn’s call were actually “highly significant” and that the former Army lieutenant general did indeed discuss sanctions with the diplomat—a revelation that made Pence “feel lied to,” according to business site Axios.

In his resignation letter, Flynn wrote that he “inadvertently briefed [Pence] and others with incomplete information.”

After serving the Obama administration as deputy attorney general, Yates was acting AG for just ten days under President Trump, who dismissed her for refusing to enforce the so-called “immigration ban” that was instituted last month. The executive order suspended the entry of all refugees to the United States for 120 days, barred Syrian refugees indefinitely, and blocked U.S. entry for citizens of seven predominantly Muslim countries for at least 90 days.

In response to the ban, Yates sent a letter to top lawyers in the U.S. Justice Department with instructions not to defend the order, a move the administration called a “betrayal.”