Former acting attorney general Sally Yates is testifying before a Senate subcommittee on May 8, and is expected to discuss her conversations with the White House about former national security adviser Michael Flynn and his contacts with Russian officials. (Reuters)

Did President Trump or anyone else in the White House know that his national security adviser was having politically troublesome, potentially illegal, 100 percent secret conversations with the Russian ambassador about lifting U.S. sanctions on Russia — sanctions imposed by President Barack Obama to punish Russia for meddling in the U.S. election to help Trump win?

That's at the heart of what a bipartisan Senate judiciary subcommittee is trying to answer Monday. Their star witness, former acting U.S. attorney general Sally Yates, could potentially help answer it by filling in the gaps of what we know and what we don't know about the early days of the Trump administration's conversations with Russia.

What we know:

About a month before President Trump was inaugurated, his national security adviser, Michael Flynn, had conversations with the Russian ambassador about potentially lifting Obama-imposed sanctions on Russia for meddling in the U.S. election once Trump was president.

Flynn originally denied that the two talked about sanctions.

After the news broke, Vice President Pence was asked about it on CBS's “Face the Nation.” Pence said: “They did not discuss anything having to do with the United States’ decision to expel diplomats or impose censure against Russia.”

The Washington Post reported in February that Flynn did talk about sanctions. Trump fired Flynn a few days later for misleading Pence.

(The Washington Post's Philip Bump has a full timeline of everything related to Flynn's downfall that's worth a read.)

How we know what we know: 

Flynn's conversations with Sergey Kislyak were picked up by the FBI during “routine surveillance” of the Russian ambassador.

Yates told the White House's lawyer, Don McGahn, that she thought Flynn was misleading them and the fact he was contradicting himself publicly could expose him to blackmail by the Russians.

Flynn was warned this would happen. In November, members of Trump's transition team said talking with the Russian ambassador would almost certainly mean Flynn would be spied on by U.S. intelligence officials — and that very fact could encourage Russia to try to take advantage of Flynn. “Officials were so concerned that Flynn did not fully understand the motives of the Russian ambassador that the head of Trump’s national security council transition team asked Obama administration officials for a classified CIA profile of Kislyak,” reported The Post on Friday.

A few days after Yates told the White House lawyer about Flynn, Trump fired Yates, the acting attorney general, for saying she would not defend Trump’s travel ban.

Each time a story about the intelligence community emerges, it includes some confusing terminology. The Fix's Amber Phillips breaks down the spy terminology you need to know. (Peter Stevenson/The Washington Post)

What we don't know:

Who else in the White House knew that Flynn wasn't telling the truth. Or, who in the White House knew that Flynn was talking about lifting sanctions on Russia all along?

Yates knew, because the FBI was collecting intelligence on whom the Russian ambassador was talking to.

That actually raises another question. Because the identity of U.S. citizens' conversations are kept secret from all but a handful of top intelligence officials, including someone in Yates's position, who unmasked Flynn's identity to the media?

Also, did the Trump White House try to block Yates from testifying to a House investigation committee in March? (The Post obtained letters from the Justice Department, now under Attorney General Jeff Sessions's control, that suggested her testimony was off-limits because of presidential communication privilege. White House press secretary Sean Spicer denied they tried to block Yates.)

White House press secretary Sean Spicer on March 28 disputed a Washington Post report about efforts by the Trump administration to prevent former acting attorney general Sally Yates from testifying before Congress. (Reuters)

The Trump administration has characterized what Yates told them about the nature of Flynn's conversations with Russia as less “alarm bells!” and more “hey, you might want to check this out.”

Here's White House Chief of Staff Reince Priebus in February on CBS's “Face the Nation”: “Our legal counsel got a heads up from Sally Yates that something wasn’t adding up with his story. And then so our legal department went into a review of the situation. . . . The legal department came back and said they didn’t see anything wrong with what was actually said.”

We'll see if Yates agrees with that characterization when she takes the witness stand Monday afternoon. Every word she shared with people in the White House about Flynn's conversations regarding lifting Russia sanctions could help answer how much, if anything, Trump and other top officials knew about it.