A GOP lawmaker accused the Internal Revenue Service of obstructing congressional investigations into the agency’s targeting of tea party groups, a charge the head of the IRS denied.

Rep. Darrell Issa, R-Calif., chairman of the House Oversight Committee, said the IRS has been slow in producing documents that are so thoroughly blacked out they are useless to investigators.

Issa said he plans to bypass IRS lawyers and will subpoena documents directly from the Treasury Department, which oversees the IRS.

“You are slow-rolling us,” Issa told acting IRS head Danny Werfel in a heated exchange during a committee hearing Friday. “There are important facts to get out, and you are obstructing.”

“That is not true,” Werfel fired back.

Werfel said that by the end of the day Friday, the IRS will have given more than 16,000 pages of documents to Issa’s committee and more than 70,000 pages to Congress as a whole. Werfel said documents are blacked out to protect confidential taxpayer information.

Issa’s committee does not have legal authority to receive confidential taxpayer information. In Congress, that authority is reserved for the chairmen of the two tax-writing committees, House Ways and Means and Senate Finance, and their designated staff.

Werfel said the two tax-writing committees are receiving full documents. However, both the House Ways and Means Committee and the Senate Finance Committee have also complained that the IRS is producing documents too slowly.

On Friday, Sen. Max Baucus, D-Mont., and Sen. Orrin Hatch, R-Utah, released an interim report on the Senate Finance Committee’s investigation. The report reaches no conclusions, but the two senators said in a joint statement, “The IRS needs to be more cooperative in providing us with the documents needed to fully carry out this investigation.”

Baucus chairs the Finance Committee and Hatch is the panel’s top Republican. They said the IRS has provided them with 21,100 pages of documents.

“I’ll tell you one thing,” Issa said, holding up a page that was completely blacked out. “As these pages, which are almost impossible to figure out where they came from, are gone through by the Ways and Means Committee, you’d better hope, you’d better really hope, that we don’t find something there that clearly should not have been redacted, which we expect we will.”

Werfel said some pages were completely redacted because they came from individual taxpayer files, which were requested by Congress.

The three congressional committees are investigating the IRS for improperly targeting tea party groups for additional scrutiny when they applied for tax-exempt status from 2010 to 2012.

The IRS acknowledged in May that agents working in a Cincinnati office had improperly singled out tea party groups for extra scrutiny. The IRS has since released documents suggesting that progressive groups may have been targeted, too.

Congressional investigations have so far shown that IRS supervisors in Washington — including lawyers in the chief counsel’s office — oversaw the processing of tea party applications. But there has been no evidence that anyone outside the IRS directed the targeting or that agents were politically motivated.

More than 100 IRS employees are working to produce documents for Congress, including 70 lawyers who are reviewing documents full time, Werfel said.

In addition, 19 employees have been made available for interviews by congressional investigators, and IRS officials have appeared at 15 congressional hearings since the scandal broke, Werfel said.

President Barack Obama appointed Werfel to run the agency on a temporary basis in May after forcing acting Commissioner Steven Miller to resign.

On Thursday, Obama nominated John Koskinen, a retired corporate and government official with experience managing numerous organizations in crisis, to a five-year term as IRS commissioner. Koskinen’s nomination is subject to Senate approval.