WASHINGTON (AP) — The Republican chairman of a powerful House committee called for the removal of the IRS commissioner Monday, saying he has obstructed congressional investigations into the treatment of conservative groups.

Rep. Jason Chaffetz, R-Utah, sent a letter Monday to President Barack Obama, asking him to remove IRS Commissioner John Koskinen. Chaffetz is chairman of the House Oversight Committee, which has been investigating the IRS for more than two years.

"Throughout his tenure, Commissioner Koskinen obstructed these congressional investigations," Chaffetz wrote in a letter that was also signed by 20 other Republicans on the Oversight Committee. "His obstruction takes the form of failure to comply with a congressional subpoena, failure to testify truthfully and failure to preserve and produce up to 24,000 emails relevant to the investigation."

Chaffetz also raised the possibility of impeaching Koskinen or voting to hold him in contempt of Congress. But when pressed, Chaffetz was noncommittal, saying those were tools available to Congress if the White House does not act.

The White House had no comment Monday, but the Treasury Department issued a statement saying Koskinen has Treasury Secretary Jacob Lew's full confidence. The IRS is part of the Treasury Department.

"As Secretary Lew has said, Commissioner Koskinen is a man of the highest integrity with a steadfast commitment to public service during difficult times," Treasury said. "His decades of experience turning around both public and private institutions continue to make him the right person to lead the IRS during a critical time for the agency."

The IRS issued a statement disputing Chaffetz's allegations, saying the agency and "Koskinen have been cooperative and truthful with the numerous investigations underway."

The IRS says more than 1 million pages of documents have been provided to Congress and the agency has participated in more than 30 congressional hearings.

"The agency will continue to cooperate with the committees, support the important oversight role of Congress as well as make additional improvements in our operations and processes," the IRS statement said.

The Senate confirmed Koskinen as head of the IRS in December 2013, months after the agency acknowledged that agents had mistreated tea party and other conservative groups when they applied for tax-exempt status. Koskinen, a specialist at turning around troubled agencies and companies, was tasked with restoring trust in an agency that had lost much of its top management to the scandal.

Shortly after the scandal erupted, Obama forced acting IRS Commissioner Steven Miller to resign.

Chaffetz said Koskinen failed to preserve IRS emails by a key figure in congressional investigations, despite promising to produce them.

Last month, the IRS inspector general told Congress that two IRS workers at a computer center in West Virginia had erased computer backup tapes that could have contained up to 24,000 emails to and from Lois Lerner, who used to head the IRS division that oversees tax-exempt groups.

The inspector general said the workers might have been incompetent, but he found no evidence that they had destroyed evidence as part of a conspiracy.

The emails were initially lost when Lerner's computer hard drive crashed in 2011.

"The bottom line is that the inspector general found no evidence to back up Republican claims of political motivation, White House involvement or intentional destruction of evidence," said Rep. Elijah Cummings of Maryland, the top Democrat on the Oversight Committee. "Calls for Commissioner Koskinen to step down are nothing more than an attempt by Republicans to manufacture a political crisis based on allegations that have already been debunked."

Lerner, who has since retired, has emerged as a central figure in congressional investigations. The House voted to hold her in contempt of Congress last year after she refused to answer questions at two House Oversight hearings.

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