WASHINGTON — IRS Commissioner John Koskinen declined Friday to apologize for years of missing employee emails, including those of former official Lois Lerner, a central figure in the controversy over the agency’s extra scrutiny of tea party groups.

“I don’t think an apology is owed,” Koskinen told a panel of lawmakers, saying the loss was attributable to technical glitches.

Testifying at a contentious House Ways and Means Committee hearing, Koskinen faced sharp criticism from Republicans, with one all but calling him a liar, while Democrats defended the agency and its boss.

The Internal Revenue Service has been embroiled in the controversy over its scrutiny of tax-exempt advocacy groups since last year, facing Republican outrage as Democrats insist the agency’s mistakes were based on bureaucratic confusion and not political motivations.

The agency alerted Congress to the missing emails June 13, raising more red flags for some.

Rep. Paul Ryan, R-Wis., noted that the IRS sat on the email information until congressional investigators specifically asked about it. The panel has requested all IRS emails related to the targeting matter.

“I am sitting here listening to this testimony – I don’t believe it,” Ryan said. “That’s your problem. Nobody believes you.”

The IRS said it lost many of Lerner’s communications when her computer crashed in June 2011. The agency said it destroyed her hard drive as a matter of protocol after trying to recover the data with help from technical experts, including IRS forensic specialists.

The agency has released emails showing that Lerner reached out for help with fixing her hard drive. “My computer skills are pretty basic, so nothing fancy – but there were some documents in the files that are irreplaceable,” Lerner wrote in July 2011 to an IT manager. “Whatever you can do to help is greatly appreciated.”

The manager later said: “I checked with the technician, and he still has your drive. He wanted to exhaust all avenues to recover the data before sending it to the ‘hard drive cemetery.’ “

The IRS has given congressional investigators thousands of emails relating to the targeting controversy and plans to provide thousands more Lerner emails that were lost from the 2009-2011 time period by searching the files of dozens of other individuals.

Rep. Sandy Levin, D-Mich., the committee’s ranking member, defended the IRS during the hearing, saying: “There is absolutely no evidence to show that Lerner’s computer crash is anything more than equipment failure.”

But Republicans insisted that the agency should have brought the issue to the committee’s attention earlier.

“You can blame it on a technical glitch, but it is not a technical glitch to mislead the American people,” said Rep. Dave Camp, R-Mich., who chairs the panel. “You say that you’ve lost the emails, but what you’ve lost is all credibility.”