Washington (CNN) -- Washington's war over alleged Internal Revenue Service targeting of conservative groups escalated sharply on Friday as Republican House Oversight Committee Chairman Darrell Issa issued a subpoena to Treasury Secretary Jack Lew, and the head of the IRS accused Issa of making a series of "inaccurate and unfair" statements earlier in the week.
At a testy hearing Friday morning -- shortly before Congress broke for its summer recess -- Issa blasted acting IRS chief Daniel Werfel for "slow-rolling" congressional investigators by refusing to release requested records in a timely manner.
"You promised to do things and you're not," Issa told Werfel. "There are important facts to get out and you're obstructing them."
"I am not. That is not true," Werfel replied.
Issa said he was "deeply disappointed" and had been left with "no choice" but to issue the subpoena requiring the handover of all e-mails involving multiple IRS officials over the past few years.
And while the conservative congressman has previously acknowledged no evidence of any Obama White House link to the matter, his subpoena also demands copies of all e-mails exchanged between White House and IRS officials since early 2010.
"While the Obama administration has so publicly deflected responsibility for the targeting, it simultaneously has attempted to thwart congressional oversight into the matter," Issa wrote to Lew.
In a letter sent to Issa on Friday, Werfel said, "I continue to strongly disagree with the characterization of the facts set forth in your July 30 letter (to me) and your statement that the IRS has attempted, in any way, to impede the on-going investigations being conducted by your committee."
Such assertions "are inaccurate and unfair," he added.
In his July 30 letter to Werfel, Issa accused the IRS of systematically trying to "delay, frustrate, impede, and obstruct" House investigators.
Issa didn't say what the consequences of the alleged obstruction would be, but noted that blocking congressional investigations is a crime punishable by up to five years in prison.
Meanwhile, the top Democrat and Republican on the Senate Finance Committee also criticized the IRS for not responding fast enough to their own investigation into alleged unfair scrutiny of conservative outfits seeking tax-exempt status.
Sen. Max Baucus, D-Montana, and Sen. Orrin Hatch, R-Utah, said their investigators have so far interviewed 14 IRS employees and reviewed over 21,000 pages of documents.
"While we are encouraged with some of the changes within the (IRS) since our investigation began, the IRS needs to be more cooperative in providing us with the documents needed to fully carry out this investigation," the senators said.
Baucus and Hatch stressed that they had not yet reached any conclusions.
For his part, Werfel insisted in his letter Friday that the "IRS is fully committed to transparency and cooperation with the ongoing review being conducted" by Congress.
"No process is perfect, but we are continually striving to improve this one," he said.
Werfel went on to say Issa's July 30 letter "contains certain misunderstandings" in need of correction.
Among other things, Werfel took issue with Issa's claim that the IRS has handed over only 12,000 of more than 64 million pages of documents initially tagged as possibly relevant to the investigation.
"There are not 64 million responsive documents," Werfel wrote. "That number referred to raw data that is largely unrelated to and unresponsive to congressional requests."
Werfel said that the massive amount of raw data is in the process of being electronically filtered to "eliminate a portion of non-responsive information." He also noted that 72% of the documents subjected to further review by IRS attorneys so far have proven "not responsive to congressional inquiries."
Werfel noted that only a "fraction" of the documents being reviewed by 70 IRS attorneys working full time on handling the requests "will ultimately be produced."
Alluding to Issa's complaint of "excessive redactions" in documents handed over to investigators, Werfel stressed that the IRS "is taking its obligations to protect the confidentiality of tax returns ... very seriously."
During the hearing, Issa said Werfel's interpretation of the statute dealing with tax privacy and redactions "is so broad that you're delivering no meaningful information."
Issa and other Republicans have insisted for months that after President Barack Obama was first elected, the IRS began intentionally targeting conservative outfits seeking tax-exempt status.
House GOP leaders have worked hard to keep the matter in the headlines, and have encouraged their members to discuss it during Congress's five-week summer recess.
Before leaving Washington on Friday, House Republicans passed a bill that would prevent the IRS from implementing or enforcing any provisions in Obama's health care reform law. The measure, which has no chance of clearing the Democratic-controlled Senate, was passed purely to help bolster the GOP's message.
Democrats insist the IRS improperly scrutinized groups on both the left and right as part of a clumsy attempt to administer unclear 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."