(WASHINGTON) — An Obama administration official who warned the Trump White House about contacts between one of its key advisers and Russia is set to speak publicly for the first time about the concerns she raised.

Former acting Attorney General Sally Yates is testifying Monday before a Senate Judiciary subcommittee investigating Russian interference in the 2016 presidential election.

The highly anticipated hearing — it is Yates’s first appearance on Capitol Hill since her firing in January — is expected to fill in key details in the chain of events that led to the ouster of Michael Flynn, President Donald Trump’s first national security adviser, in the early weeks of the administration.

The February resignation followed media reports that Flynn had discussed U.S.-imposed sanctions on Russia with Ambassador Sergey Kislyak during the presidential transition period, which was contrary to the public representations of the White House.

Yates is likely to testify Monday that she warned White House counsel Don McGahn on Jan. 26 that Flynn’s contacts — and the discrepancies between what the White House said happened on the calls and what actually occurred — had left him in a compromised position, according to a person familiar with her expected statements. The person was not authorized to discuss the testimony by name and requested anonymity.

White House officials have said publicly that Yates merely wanted to give them a “heads-up” about Flynn’s Russian contacts, but Yates is likely to testify that she expressed alarm to the White House about the incidents, according to the person.

Trump has said he has no nefarious ties to Russia and isn’t aware of any involvement by his aides in Moscow’s interference in the election. He’s dismissed FBI and congressional investigations into his campaign’s possible ties to the election meddling as a “hoax” driven by Democrats bitter over losing the White House. He’s also accused Obama officials of illegally leaking classified information about Flynn’s contacts with Kislyak.

Also scheduled…