Former acting Attorney General Sally Yates is expected to testify next week before a Senate Judiciary subcommittee, and according to multiple congressional sources, as well as law enforcement sources, her testimony is expected to be among the most important because it will reveal what she told the White House about former National Security Adviser Michael Flynn. Originally scheduled to testify last month, her appearance before the House Intelligence Committee was abruptly cancelled days before by then-Chairman Devin Nunes, who was in the midst of chasing down information that backed up President Trump's wiretapping claim.

At the time, House and Senate Democrats were eager to hear Yates testify in an open hearing rather than a closed session.

Deputy Attorney General Sally Yates speaks during a news conference at the Justice Department in Washington on June 28, 2016. AP

According to multiple sources, when Yates testifies she is expected to reveal that she believed former National Security Adviser Michael Flynn was "compromised."

Get Breaking News Delivered to Your Inbox

Yates notified the White House that Flynn had not been forthcoming about his statements to Vice President Pence about his contacts with Russian Ambassador Sergey Kislyak during the transition period. Flynn had told Trump officials, including Pence, that he had not talked to Kislyak about U.S. sanctions against Russia -- as the Obama administration was implementing those sanctions.

Flynn was later fired because he misled Vice President Pence. However, there was a two-week delay between when the president fired Flynn and the time period in which Yates notified the White House.

Congressional sources say they are interested in knowing why the White House was so slow to react. During that period, the FBI counterintelligence investigation into whether Trump campaign officials were in collusion with the Russians was well underway and Flynn – as national security adviser -- was handling some the most sensitive information.

In the ensuing weeks Flynn confirmed by filing official documents that he was a registered foreign agent during the period, which according to the law, is something that he should have revealed much earlier.

Yates was later fired by Mr. Trump after ordering DOJ attorneys to stop defending his original travel ban.