(CNN) -- In a knife fight of a congressional hearing, IRS Commissioner John Koskinen defended his agency, but he sparked an uproar from Republicans by admitting it destroyed a key hard drive wanted in an investigation, and he did not apologize for the data loss.
"I don't think an apology is owed, not a single email has been lost since the start of this investigation," Koskinen told a dais full of skeptical GOP members on Friday.
He argued the IRS has done its best to find the emails that originally went missing in 2011.
Those emails are central to an investigation of why the agency targeted tea party and other political groups for extra scrutiny when they applied for tax-exempt status.
The hearing by the Republican-led House Ways and Means Committee focused on the week-old revelation from the IRS that a hard drive belonging to a former top agency official at the center of the congressional probe, Lois Lerner, crashed in 2011. Tens of thousands of her emails saved on it dating to 2009 were destroyed.
Lerner and that time frame are critical in the investigation.
The targeting issue has its roots in the 2010 rise of the tea party and the Supreme Court Citizen's United decision, which bolstered the ability of independent groups to engage in political activity.
Lerner ran the division in charge of tax-exempt status and is the highest-ranking official directly connected to the targeting so far.
She has since retired and refused to testify before Congress on the matter, citing her constitutional rights against having to do so. The House charged her with contempt of Congress for her refusal to speak.
IRS pushes back
Koskinen told lawmakers on Friday that not only has the agency tried to retrieve the Lerner emails, but he also pushed back at Republican reports that emails of six other IRS employees, including a former chief of staff, are missing.
"It turns out ... that none of (the chief of staff's) emails appear to be lost," Koskinen said, scolding the Republicans on the committee. "Piecemealing out information about a possible problem simply results in press releases and angry letters to me."
For their part, Republicans were blistering. They expressed frustration that has built up for the past week.
"The American people have no reason to trust the IRS ... on this issue," said Ways and Means Committee Chairman Dave Camp.
The Michigan Republican pressed Koskinen on whether his agency knows exactly where the destroyed hard drive ended up.
He replied that he was not sure if there was a serial number attached to the hard drive and that it was destroyed three years ago.
Republicans drilled down on when the IRS knew about the hard drive problems.
Time line key
The time line from Koskinen was nuanced. He said the agency knew in February that there was some kind of problem with Lerner's hard drive, but that it took until late April or May to determine that the hard drive crash had erased data.
Why didn't he tell Congress sooner?
Koskinen responded that the agency was trying to retrieve as many lost emails as possible first but decided to notify lawmakers after receiving a letter from the Senate Finance Committee last Friday.
The committee indicated that it had completed its investigation and was doing a final check to make sure the IRS had sent every available Lerner email.
Koskinen also pointed out that emails about the Lerner hard drive crash had been included in large bundles of documents, tens of thousands at a time, that the agency sent to congressional committees last fall.
"We've provided the information," Koskinen began to say under questioning by Texas Republican Kevin Brady.
"You have not provided us the information!" Brady blasted back. "At this point why should anyone believe you? ... This is the most corrupt and deceitful IRS in the history!"
Not to be believed
The outcry continued, leading to one of the most contentious exchanges of the day.
"This is unbelievable!" exclaimed a visibly irate Rep. Paul Ryan, a Wisconsin Republican. "The apology should be to the American people. I don't believe it. That's your problem. No one believes you."
Koskinen, whose tone was sharp and defensive at times, responded that he has had a long career in government.
"And that is the first time anybody has said they do not believe me," he said.
"I don't believe you," Ryan repeated.
"That's fine," Koskinen said. "We can have a disagreement."
Democrats defended the IRS as an agency overloaded with work and underfunded by Congress, and accused Republicans of turning the hearing into a political tribunal.
"Was (Lerner's) hard drive crash a conspiracy? No," said Rep. Sander Levin of Michigan, the top Democrat on the panel. "The prevailing conspiracy in this matter is that of the Republican's desire to stir their base, tie the problem to the White House and keep up this drumbeat until the November election."
At one point Democratic Rep. Xavier Becerra of California gave the IRS leader friendly advice to take a deep breath before speaking.