The country’s top investigator seemed to be in the dark Thursday when pressed to provide details of the IRS investigation into the tax agency’s targeting of Tea Party and conservative groups.

Rep. Jim Jordan, R-Ohio, seemed to rattle FBI Director Robert Mueller for not knowing the specifics surrounding the IRS probe.

“You’ve had a month now to investigate,” Jordan said. “This has been the biggest story in the country and you can’t even tell me who the lead investigator is. You can’t tell me the actions the inspector general took which are not typically how investigations are done. You can’t tell me if that’s appropriate or not. This is not speculation. This is what happened.”

Mueller repeatedly declined to answer Jordan’s questions, saying he couldn’t because the investigation was ongoing or that he’d have to get back to the lawmakers with answers.

When Jordan asked again,” Can you tell me who the lead investigator is?” Mueller responded, “Off the top of my head, no.”

The day didn’t go much better for the outgoing FBI chief. He was grilled for hours by lawmakers on a number of different topics, including the federal government’s surveillance programs, the Benghazi scandal and the Boston Marathon bombings.

Mueller defended the government's collection of millions of U.S. phone records, emails and other information as vital to the nation's national security.

Early in the hearing, Mueller tried to make the case for the National Security Agency surveillance programs and said that law enforcement “must stay a step ahead of criminals and terrorists” while still heeding the civil liberties of Americans.

Mueller, who is stepping down from his post in September, said that if the metadata collection program had been in place before the Sept. 11, 2001, terror attacks, they would have identified one of the 9/11 hijackers in San Diego and most likely derailed the plot.

But Judiciary Committee Chairman John Conyers Jr. said he was “not persuaded that the argument makes it OK to collect information on every call,” adding, that by Mueller’s interpretation, it would be “anything and everything goes” situation.

Mueller also testified that the government’s controversial surveillance programs that recently surfaced complied “in full with U.S. law and with basic rights guaranteed under the Constitution.”

The Justice Department revealed last month that it had secretly gathered emails of Fox News correspondent James Rosen and phone records of The Associated Press in an effort to crack down on leakers of classified information.

The department later acknowledged that Attorney General Eric Holder was on board with a search warrant for Rosen's personal emails, obtained after federal officials accused him in an affidavit of being a likely criminal "co-conspirator" under a wartime law known as the Espionage Act.

Authorities also obtained phone records for Fox News lines, including those for a number that matched the number of Rosen's parents.

In the past week, a 29-year-old contractor leaked National Security Agency documents on the agency's collection of millions of U.S. phone records and the NSA's collection of emails and other information that people transmit online to and from foreign citizens.

That has touched off a national debate over whether the Obama administration, in its efforts to thwart terrorism, has overstepped by using intrusive surveillance methods.

Rep. Bob Goodlatte, R-Va., the committee's chairman, said when it comes to national security leaks, it's important to balance the need to protect secrecy with the need to let the news media do its job.

Goodlatte also said the committee planned to find out more about the status of what the congressman called the FBI's "stalled investigation" into the attack in Benghazi, Libya that killed four Americans, including the U.S. ambassador.

As for the Boston Marathon bombings, committee members want to know whether there was a breakdown in information-sharing between federal agencies, preventing the FBI from thwarting the explosions that killed three people and injured more than 260.

The Associated Press contributed to this report.