The top U.S. official in charge of archiving federal records testified Tuesday that the IRS ran afoul of the law by neglecting to tell his office that a trove of emails from the woman at the center of the targeting scandal disappeared after an apparent hard drive crash.

Archivist of the U.S. David Ferriero, speaking before the House Oversight and Government Reform Committee, made clear that federal agencies are supposed to report whenever their records are destroyed or even accidentally deleted. But he said that after emails from embattled IRS official Lois Lerner vanished after a computer failure in 2011, nobody told the National Archives.

"They did not follow the law," Ferriero said.

The testimony comes as lawmakers dig for answers as to how Lerner's emails disappeared and why they only found out about it earlier this month. Lerner herself twice has refused to answer questions before Congress, leaving lawmakers seeking answers from other officials who interacted with her or were involved in reviewing her documents.

Also on the witness panel was White House attorney Jennifer O'Connor, whom Chairman Darrell Issa compelled to testify via subpoena.

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Republican lawmakers, though, struggled to get answers from O'Connor, who previously worked at the IRS, about the emails. She said she only found out the "week before last" that emails were gone.

"I didn't hear that any of Ms. Lerner's emails were missing," she said.

The hearing, like several recent sessions on the IRS targeting scandal, was tense from the start.

At the beginning of the hearing, Issa accused O'Connor of being a "hostile witness" when she did not immediately answer one of his questions.

"I'm not a hostile witness," she retorted.

"Yes, you are," Issa said. He later clarified to say she is "non-cooperative."

O'Connor was brought before the committee because of her work with the IRS in the latter half of 2013, when she was brought on to help the agency respond to congressional requests for documents pertaining to the IRS targeting scandal.

Her appearance itself was controversial. The White House initially blocked her from appearing. Issa then issued a subpoena late Monday, and the White House reversed course, making her available to testify.

Republicans say the White House has not been helpful in efforts to investigate the IRS scandal. "They haven't done a damn thing to get to the truth of what happened," House Speaker John Boehner said Tuesday.

But as for O'Connor's treatment, Rep. Elijah Cummings, D-Md., Issa's Democratic counterpart on the committee, blasted the chairman for the "unilateral subpoena."

"Why is she here?" Cummings asked. "It's not because of her old job -- it's because of her new one."

The hearing was held on the heels of a Monday night hearing before the same committee, where IRS Commissioner John Koskinen testified and faced heated questions from lawmakers.

At that hearing, Issa told Kosninen: "We have a problem with you and you have a problem with credibility."

Koskinen appeared at the rare evening session to answer questions about the lost emails from Lerner, a key figure in the committee's probe into the agency's targeting of conservative groups. The agency claims the emails were lost in a 2011 computer crash.

In a heated back-and-forth with Issa, Koskinen said he had fulfilled his promise to the committee to provide it with all of Lerner's emails and that there was no way to recover ones the agency said were lost in 2011.

"If you have a magical way for me to do that, I'd be happy to hear about it," Koskinen sarcastically told Issa.

"I've lost my patience with you," Issa shot back.