A congressional hearing Friday into how the Internal Revenue Service lost thousands of emails from an ex-official accused of targeting conservative groups turned into an angry shouting match, with Republicans accusing the IRS commissioner of lying to Americans.

“This is unbelievable," Rep. Paul Ryan, R-Wis., angrily told IRS Commissioner John Koskinen. That’s your problem. Nobody believes you.”

Koskinen responded, “I have a long career. That’s the first time anyone’s said I don’t believe you.”

"I don't believe you," Ryan shot back again.

Koskinen set a defiant tone during his testimony before the House Ways and Means Committee, telling lawmakers he felt no need for the agency to apologize amid accusations of a cover-up in the targeting scandal of conservative groups.

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Republican lawmakers had demanded the emails between ex-IRS official Lois Lerner and other government officials - including some at the White House - be turned over to determine whether there was a coordinated effort to stymie conservative groups prior to the 2012 elections.

“I don’t think an apology is owed,” Koskinen said. “We haven’t lost an email since the start of this investigation.”

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That didn’t sit well with Chairman David Camp, R-Mich., who pressed the commissioner on the timeline of events and accused the agency of “keeping secrets.”

During the roughly four-hour hearing, Koskinen said his office was still attempting to recover data and that it was too soon to know exactly how many emails were missing.

GOP lawmakers are furious after learning a week ago that many Lerner emails from a two-year period supposedly have disappeared. Committee Republicans now say that the IRS may have known about this for months, and that the agency may also have lost emails from another six employees.

“The IRS is in charge of hundreds of millions of taxpayers' information. And you’re now saying your technology system was so poor that years' worth of emails are forever unrecoverable?” Camp asked. “How does that put anyone at ease? How far would the excuse 'I lost it' get with the IRS for an average American trying to file their yearly taxes who may have lost a few receipts?”

Following the hearing, Camp told reporters he thinks Koskinen still “owes a huge apology but that shows the arrogance we’re dealing with.”

The tone and exchanges between lawmakers and the commissioner frequently became heated.

Rep. Sander Levin, D- Mich., backed the IRS Friday and likened the investigation and calls of a cover-up to a political witch hunt brought on by Republicans who, he claimed, will try “to tie the problem to the White House” and will “keep up this drumbeat until the November election.”

During the testy exchange between Ryan and Koskinen, Levin tried to intervene.

“Will you let him answer the question?” Levin asked Ryan.

Ryan responded angrily, “I didn’t ask him a question!”

Levin chided his colleagues that, “witnesses deserve some respect.”

Earlier this week, Sen. Orrin Hatch, R-Utah, said he's been told that Lerner's hard drive was simply destroyed.

"They just got rid of it," he told Fox News. "It really looks bad and I've got to say it looks like a cover-up to me."

Hatch and Sen. Ron Wyden, D-Ore., are leading a bipartisan investigation in the Senate Finance Committee into the targeting scandal, separate from the House Ways and Means probe.

House and Senate Republicans, though, have common questions for the commissioner and the rest of the agency.

Hatch fired off a letter to Koskinen on Thursday voicing concern that although he met with him on Monday, the commissioner and his staff did not mention that emails from six other employees might be missing.

Lerner, the former IRS official at the center of the investigation, invoked her Fifth Amendment right at least nine times to avoid answering lawmakers’ questions. According to an audit by the Treasury Department inspector general for tax administration, Lerner did not learn that IRS staffers were improperly reviewing applications of Tea Party and other conservative groups for tax-exempt status until weeks after her computer crashed.

Lerner's computer crashed sometime around June 13, 2011, according to emails provided to Congress. She first learned about the Tea Party reviews on June 29, according to the inspector general.

Koskinen told Congress that Lerner's hard drive was unavailable to them because it had been recycled.

The IRS said last week it became aware of the missing emails in February of this year. The IRS did not know whether the other computer crashes have resulted in lost emails as well. It will also not say how often its computers fail and lose data.

The lost emails have even raised questions from the government's records officer. In a June 17 letter to the IRS, Paul Wester Jr. asked the agency to investigate the loss of records and whether any disposal of data was authorized. Wester, the chief records officer at the National Archives and Records Administration, was responding to the IRS' June 13 disclosure of Lerner's lost emails.

Wester's letter did not address the lost records of six other employees that the IRS disclosed that day. Wester said the IRS is required to report its finding within 30 days. Federal agencies are supposed to report destruction of records -- whether accidental or intentional -- to the National Archives "promptly" after an incident.

The IRS said that after Lerner's computer crashed in June 2011, technicians were not able to retrieve data from her hard drive.

In May, more than two months after the IRS discovered the emails were missing, the IRS assured Camp that it would provide all applications from groups seeking tax-exempt status in 2010 and 2011, including all files, correspondence and internal IRS records related to them. Camp had asked for the records in May 2012.

It was unclear why the IRS did not attempt to recover the emails from backup servers in June 2011, especially since Lerner told an IRS computer technician in a July 2011 email, "There were some documents in the files that are irreplaceable."

Shawn Henry, the FBI's former cyber director, said technicians should have been able to retrieve data from the servers around the times the computers crashed.

"If they knew there was a problem in 2011," said Henry, now president of CrowdStrike, a security technology company, "they could have or should have been able to recover it."

The IRS told Congress last week that recovering emails has been a challenge because doing so is "a more complex process for the IRS than it is for many private or public organizations."

The IRS was able to find copies of 24,000 Lerner emails from between 2009 and 2011 because Lerner had sent copies to other IRS employees. Overall, the IRS said it was producing 67,000 emails to and from Lerner, covering 2009 to 2013. The agency said it searched for emails of 83 people and spent nearly $10 million to produce hundreds of thousands of documents.

At the time that Lerner's computer crashed, IRS policy had been to make copies of all IRS employees' email inboxes every day and hold them for six months. The agency changed the policy in May 2013 to keep these snapshots for a longer, unspecified amount of time. Had this been the policy in 2011, when at least two of the computer crashes occurred, there likely could have been backups of the lost emails today.

The chief executive for an email-archiving company, Pierre Villeneuve of Jatheon Technologies, said most public and private sector organizations keep emails for several years, not six months, because of financial regulations and inexpensive computer storage.

The IRS has said technicians sent Lerner's hard drive to a forensic lab run by the agency's criminal investigations unit. But the information was not recoverable, a technician told her in an Aug. 5, 2011, email.

The Associated Press contributed to this report.