Washington (CNN) -- The Republican congressman spearheading a House probe of alleged Internal Revenue Service targeting of conservative political groups accused the head of the IRS on Tuesday of obstructing his panel's investigation.
California GOP Rep. Darrell Issa, chairman of the House Oversight and Government Reform Committee, warned acting IRS chief Daniel Werfel that if "the IRS continues to hinder the committee's investigation in any manner, the committee will be forced to consider use of compulsory process."
Issa did not elaborate on exactly what steps his panel's Republican majority may be prepared to take, though he noted that impeding congressional investigators could result in a prison term of up to five years.
"Obstructing a congressional investigation is a crime," Issa stressed in a letter co-signed by Rep. Jim Jordan, R-Ohio.
"Despite your promise to cooperate fully with congressional investigations, the actions of the IRS under your leadership have made clear to the committee that the agency has no intention of complying completely or promptly with the committee's oversight efforts," Issa said.
"The systematic manner in which the IRS has attempted to delay, frustrate, impede, and obstruct the committee's investigation raises serious concerns about your commitment to full and unfettered congressional oversight," Issa wrote.
Responding to Issa's claim, an IRS spokeswoman told CNN the agency is "aggressively responding to the numerous data requests we've received from Congress."
"We are doing everything we can to fully cooperate with the committees, and we strongly disagree with any suggestions to the contrary," Michelle Eldridge said.
Among other things, Issa claimed the IRS has handed over only 12,000 of the more than 64 million pages of documents initially identified as potentially relevant to the investigation into the alleged unfair targeting of conservative outfits seeking tax-exempt status.
"This incredibly slow pace of production has been an unnecessary attempt to frustrate the committee's oversight efforts," he said.
Edridge, in turn, argued that "while the volume of raw data collected ... is quite high, it is a misleading figure to use in order to determine the volume of material the IRS will ultimately produce."
"The vast majority of it is completely unrelated to the congressional investigations," she said. "Once the data is limited to the time period in question, and the issue in question, we expect the final tally of produced documents will be far lower -- in the neighborhood of 460,000 documents or fewer."
Eldridge said 70 of roughly 1,500 attorneys in the IRS chief counsel's office are currently working full time to respond to congressional inquiries into the matter.
It is a "time and labor intensive review process," she said.
For his part, Issa also complained that documents produced by the IRS "contain excessive redactions that go well beyond those necessary to protect confidential taxpayer information."
Furthermore, Issa asserted that a senior IRS official -- Cindy Thomas -- had been "affirmatively prevented" from providing congressional investigators with relevant documents in her possession.
Additionally, the chairman blasted the IRS for allegedly trying to "carefully orchestrate the public release" of information contained in a 30-day review of the matter back in June -- before providing the information to the committee.
Issa and other Republicans have insisted for months that after President Barack Obama was first elected, the IRS started unfairly targeting conservative outfits seeking tax-exempt status.
Democrats, however, argue the IRS improperly scrutinized groups on both the left and right as part of a clumsy attempt to administer vague election-related tax laws.
At a speech in his home state of Illinois last week, Obama ripped what he labeled Washington's "endless parade of distractions, political posturing and phony scandals."
The controversy has been the subject of numerous congressional hearings. And on Monday, Issa and Jordan argued for a new investigation -- this time into alleged IRS targeting of conservative groups that already have tax-exempt status.
Maryland Rep. Elijah Cummings, the top Democrat on Issa's panel, called the information underlying the new assertion "partial and incomplete," and designed to fit a pre-existing "political narrative."
House GOP leaders have indicated they intend to keep publicly pressing on the issue this week, voting on a proposal to prevent the IRS from implementing or enforcing any provisions in the president's health care reform law.
Top House Republicans have also suggested their members highlight the matter during the upcoming August congressional recess.