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Lawmakers pummeled the IRS on Thursday for spending millions of the taxpayer dollars it collected on lavish conferences, with one Republican calling the behavior "maliciously self-indulgent."

The IRS was summoned to a House oversight hearing to explain how it blew through $50 million on conferences between 2010 and 2012, including spending more than $4 million on a single California conference in 2010. For the agency's many critics, it compounded frustrations which had already mounted over the agency's targeting of conservative groups.

"Not only does the IRS take your money, not give you proper answers, but then when it comes to tens of millions of dollars, use it in a way that is, at best, maliciously self-indulgent," said Rep. Darrell Issa, R-Calif., chairman of the House Oversight and Government Reform Committee.

Democratic Rep. Elijah Cummings, of Maryland, likewise slammed the agency, specifically for spending $50,000 on "ridiculous" training videos, one of which was a "Star Trek" parody. Cummings noted that's more than many families make in a year.

Inspector General J. Russell George ticked off a list of over-the-top expenses incurred at the 2010 conference in Anaheim. He said the agency spent $35,000 on planning trips, $64,000 on gifts and trinkets including squirting fish toys, and $135,000 on outside speakers.

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Faris Fink, the head of the IRS division that staged the conference and was the deputy in 2010 when he attended, apologized on Thursday for the expenses.

He said the conference was held to help train agency managers, but acknowledged it was "not the best use of taxpayer dollars."

Fink has the distinction of playing Mr. Spock in a cheesy but slickly produced "Star Trek" video that IRS employees filmed for the conference.

Fink also stayed in a room that normally cost $1,499 a night, the IG report said. A total of 132 IRS officials received room upgrades at the conference. The tax agency paid a flat daily fee of $135 per hotel room, the report said, but the upgrades were part of a package deal that added to the overall cost of the conference.

The training conference in Anaheim was held for 2,609 managers in the IRS' small business and self-employed division.

At the conference, the commissioner of the division, Christopher Wagner, stayed in a presidential suite that normally cost $3,500 a night, according to the inspector general's report. Wagner became chief of the IRS office of appeals in 2011 and retired this year.

The agency has imposed strict regulations to prevent expensive conferences in the future. And on Wednesday, the new acting head of the agency placed two officials on administrative leave for accepting free food at a party in a private suite at the IRS conference in 2010.

Pending a review, the two officials could lose their jobs, the agency said.

"When I came to IRS, part of my job was to hold people accountable," acting IRS Commissioner Danny Werfel said in a statement. "There was clearly inappropriate behavior involved in this situation, and immediate action is needed."

The Associated Press contributed to this report.