WASHINGTON — Sally Yates largely avoided the public spotlight when she served as deputy attorney general during the final two years of the Obama administration.

That all changed — dramatically — in the 10 days she spent as acting attorney general during the infancy of the Trump administration.

The career federal prosecutor from Georgia, a holdover from the Obama Justice Department, was abruptly fired Jan. 30 for refusing to defend the new president’s controversial travel ban in court.

Yet it is the extraordinary warning she delivered just four days earlier to the White House counsel’s office about former Trump national security adviser Michael Flynn that is the reason for her testimony to Congress on Monday.

It's perhaps the most-anticipated congressional hearing since FBI Director James Comey publicly acknowledged last month that the bureau was investigating possible collusion between Trump campaign associates and Russian officials.

For the first time publicly, Yates is expected to recount to a Senate panel a Jan. 26 meeting in which she alerted White House Counsel Don McGahn that Flynn had lied to administration officials, including Vice President Mike Pence, about his conversations with Russian ambassador Sergey Kislyak before Trump's inauguration. Those misrepresentations, Yates would maintain, made the former Army lieutenant general vulnerable to possible blackmail.

Specifically troubling, according to officials who have previously described the communications, was that Flynn had talked with the ambassador about sanctions imposed by the Obama administration. Those conversations were secretly monitored by federal authorities, as are most communications involving foreign diplomats.

Pence, meanwhile, had said he had been assured by Flynn that the subject of sanctions was not raised in the Kislyak conversations.

Shortly after Yates' warnings were made public, Flynn was forced to resign, ending the shortest tenure of any president's national security adviser – while stoking further suspicion about the ties between Trump associates and Russian officials.

Since then, Flynn and former Trump advisers, including Carter Page, Paul Manafort and Roger Stone have become the subjects of fresh scrutiny about their Russian ties.

On Saturday, the Washington Post reported that Trump transition team members had warned Flynn about the risks of communicating with the Russian ambassador, prior to the his December telephone conversations with Kislyak. According to the report, Flynn was told in November that the ambassador's communications were likely being monitored by U.S. authorities.

Separately, the Senate Intelligence Committee, which along with the House Intelligence Committee are conducting a parallel inquiries into Russian interference in the 2016 elections, recently asked the advisers to provide information about their activities.

Flynn also is now under investigation by the Pentagon Inspector General for failing to inform Defense Department officials about seeking payments from foreign governments.

Sen. Lindsey Graham, R-S.C., who is leading Monday's Senate Judiciary subcommittee hearing, has said the panel's examination is necessary to "hold [Russia] accountable.''

"Based on evidence presented by our intelligence and law enforcement communities, I believe Russia interfered in our election,'' Graham has said. "I do not believe it changed the outcome, but I have no doubt they interfered.'' The U.S. intelligence community has accused Moscow of orchestrating a campaign of cyberattacks to hack Democratic political organizations and release stolen information to undermine confidence in the American election.

It is not immediately clear how much new information Yates and former Director of National Intelligence James Clapper, who also is scheduled to testify Monday, will be able provide. Both witnesses are likely to be constrained by the classified nature of the information surrounding the events they were privy to.

In March, when Yates' attorney had notified the Justice Department and White House of her intent to appear at a previously scheduled House Intelligence Committee hearing, the attorney was warned that Yates' testimony could contained privileged communications that might be barred.

Ultimately, Yates' scheduled March 28 appearance was canceled.

Trump spokesman Sean Spicer has since indicated that the White House has no objection to Yates' testifying. And last week , Graham signaled that senators will ask Yates about what other possible considerations drove her to alert the White House counsel about her concerns for Flynn.

Responding to a series of questions from Graham, FBI Director James Comey recalled in separate testimony last Wednesday before the Senate Judiciary Committee that he met with Yates to discuss her concerns about Flynn's activities. The meeting occurred after FBI agents interviewed Flynn about his conversations with the Russian ambassador.

Comey declined to elaborate on the meeting with Yates in the open committee session.

Attorneys for Yates and Flynn did not respond to inquiries seeking comment.

Flynn, meanwhile, has sought immunity from any possible prosecution. Both the House and Senate Intelligence committees, in the midst of continuing Russia probes, have indicated that it is too early in their investigations to cut a deal for Flynn's testimony. Separately, preliminary discussions about Flynn's prospects for immunity in the FBI investigation also have yielded no agreement.