Former acting Attorney General Sally Yates testifies Monday before a Senate subcommittee.

WASHINGTON — Former acting Attorney General Sally Yates said she bluntly warned the Trump White House in January that new National Security Adviser Michael Flynn “essentially could be blackmailed” by the Russians because he apparently had lied to his bosses about his contacts with Moscow’s ambassador in Washington.

The congressional testimony Monday from Yates, an Obama administration holdover fired soon after for other reasons, marked her first public comments about the concerns she raised and filled in basic details about the chain of events that led to Flynn’s ouster in February.

Her testimony, coupled with the revelation hours earlier that President Barack Obama himself had warned Donald Trump against hiring Flynn shortly after the November election, made clear that alarms about Flynn had reached the highest levels of the U.S. government months before. Flynn had been an adviser to Trump and an outspoken supporter of his presidential candidacy in the 2016 campaign.

Yates, appearing before a Senate panel investigating Russian interference in the election, described discussions with Don McGahn, the Trump White House counsel, in which she warned that Flynn apparently had misled the administration about his communications with Sergey Kislyak, the Russian ambassador.

“We felt like it was critical that we get this information to the White House, in part because the vice president was unknowingly making false statements to the public and because we believed that Gen. Flynn was compromised with respect to the Russians,” Yates said.

“To state the obvious,” she added later, “you don’t want your national security adviser compromised with the Russians.”