In a heated Senate committee hearing on Wednesday, former deputy attorney general Sally Yates testified that former Trump national security adviser Michael Flynn secretly “neutered” President Obama’s punitive actions toward Russia in late 2016.

Yates, who testified remotely before the Senate Judiciary Committee, served in the Justice Department at the end of the Obama administration and in the earliest days of Trump’s, until she was being fired for insubordination for refusing a presidential order.

Committee chairman Sen. Lindsey Graham (R-SC) has been sharply critical of the FBI’s handline of the so-called “Russiagate” investigation of Moscow’s meddling in the 2016 U.S. presidential election, on behalf of the Trump campaign.

“Seeking to use Yates to discredit the FBI’s investigations … Republicans instead got a spirited defense of that work as ethical and necessary, even though she was critical of some of the FBI’s moves at the time,” reports the Washington Post.

Some observers on Twitter indicated that Graham seemed more or less satisfied with Yates’s testimony. But even before the hearing opened, Trump couldn’t resist firing off a tweet attacking her credibility.

Yates said the FBI investigation of Flynn, a retired Army lieutenant general, was triggered by Flynn’s “undercutting” of Obama policies and then “covering it up” by lying to investigators.

On Wednesday, Graham told Yates that she was trying to criminalize “policy differences” and insisted Flynn was prosecuted “because they [Russiagate investigators] hate his guts.”

“No, senator,” she responded. “We weren’t investigating a policy difference,” adding that they were “investigating a counterintelligence threat.”

“If Gen. Flynn didn’t think he was doing anything problematic, then he wouldn’t have needed to do anything to cover it,” Yates said of Flynn’s false statements about his conversations with the then-Russian ambassador in late 2016, reports NPR.

Yates said Flynn’s lies to the FBI were “absolutely material to a legitimate investigation,” NPR says.

Flynn resigned under pressure after barely 3 weeks in the Trump White House and later in 2017 agreed to a deal with special counsel Rober Mueller to plead guilty to “willfully and knowingly” lying to the FBI — and to cooperate with Mueller’s investigation.

Early this year, Flynn tried to withdraw his guilty plea, and the Trump Administration’s Justice Department said it would drop all charges against him.

A federal judge blocked that action, putting the matter on hold.