IRS Commissioner John Koskinen on Friday declined during testimony to apologize for years’ worth of missing e-mails to and from IRS employees, including former official Lois Lerner, a central figure in the agency’s tea party controversy.

“I don’t think an apology is owed,” Koskinen said at a hearing with the House Ways and Means Committee, which is investigating the agency’s targeting of nonprofit advocacy groups based on their names and policy positions. He said the loss was attributable to “technical glitches.”

IRS Commissioner John Koskinen testifies during a hearing before the House Ways and Means Committee on Friday on Capitol Hill. (Photo by Alex Wong/Getty Images)

Republicans on the panel criticized Koskinen for not being more forthcoming about the missing e-mails, which the Internal Revenue Service brought to the committee’s attention on Friday, despite knowing about the issue for months.

Rep. Paul Ryan (R-Wis.) complained that the agency sat on the information until congressional investigators specifically asked about it. The panel has requested all IRS e-mails 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 subsequently destroyed her hard drive as a matter of protocol after trying to recover the data with help from technical experts, including IRS forensic specialists.

MORE: Here’s how the IRS lost emails from key witness Lois Lerner

The agency has released e-mails 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, but Republicans are not satisfied that the agency has provided everything. The IRS said it plans to provide thousands more Lerner e-mails 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 Friday’s 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 e-mails, but what you’ve lost is all credibility.”

Koskinen is scheduled to testify again Monday at a hearing with the House Oversight and Government Reform Committee.

Rep. Darrell Issa (R-Calif.), who heads the panel, has called on former IRS counsel Jennifer O’Connor, who now works as an attorney for the White House, to appear at the same hearing. Congressional investigators have learned that she played a key role in the agency’s handling of records requests related to the targeting issue.