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The chairman of the House Oversight and Government Reform Committee threatened to hold the IRS commissioner in contempt of Congress if he does not turn over emails from embattled ex-IRS official Lois Lerner and others.

In another heated congressional hearing on the IRS targeting scandal, Rep. Darrell Issa, R-Calif., on Wednesday questioned IRS boss John Koskinen on why his agency still has not produced documents subpoenaed by his committee. The chairman wants to see all of Lerner's and other agency officials' emails since 2009, to look for political motivations that might have been behind the practice of singling out conservative groups applying for tax-exempt status. The last subpoena went out Feb. 14.

"We issued you a lawful subpoena," Issa told the commissioner, adding that delivering all emails from the four officials flagged is "not difficult."

Koskinen repeatedly pledged, in general, to "continue to work with you" on the investigation.

Issa, seemingly frustrated by the responses, said he expects Koskinen to comply "or potentially be held in contempt."

Issa is also weighing whether to pursue contempt proceedings against Lerner herself, for refusing to testify about her role in the targeting -- Lerner is the former head of the Exempt Organizations Division.

Koskinen, though, appeared to agree to eventually turn over the documents, while warning that many of the emails would be "irrelevant" to the investigation.

"If you want them all, we'll give them all to you," the commissioner said. He said it could take "years, not months."

The hearing bared the frustration among both Republican lawmakers and IRS officials over the course of the multiple investigations into the targeting scandal.

Koskinen, in his opening testimony, said the agency has devoted 250 employees and millions of dollars to the six separate investigations -- and that they have produced hundreds of thousands of pages of documents so far.

He noted that his team would still have to go through the subpoenaed emails and redact them for sensitive information. "A significant part of it's going to be irrelevant," he said.

But Republicans argue they need to see Lerner's emails in part because she refuses to testify before the committee. Earlier this month, Lerner invoked her Fifth Amendment right not to answer questions before the House oversight panel for a second time. Issa and the committee's top Democrat, Maryland Rep. Elijah Cummings, also clashed at that hearing after Issa cut off his microphone.

Democrats continued to charge Wednesday that Republicans are using the IRS probe for political gain.

"This is all feeding the base," Rep. Gerry Connolly, D-Va., said.