FBI Director Robert Mueller appeared Tuesday at a House Judiciary Committee oversight hearing, where he was asked about the bureau’s recently concluded anthrax investigation.

Mueller was questioned on the investigation both at the hearing and in a letter from Rep. Jerrold Nadler (D-N.Y.), who chairs the panel’s Subcommittee on the Constitution, Civil Rights and Civil Liberties, Judiciary Chairman John Conyers (D-Mich.) and Rep. Bobby Scott (D-Va.), who chairs the Subcommittee on Crime, Terrorism and Homeland Security.

In the letter dated Sept. 5, they asked Mueller to address questions about whether the White House had pressured the FBI to pursue a connection between the attacks and al-Qaida; why government scientist Steven Hatfill, at one point considered a suspect, had remained under suspicion even after evidence showed he was not responsible; and about why Bruce Ivins, an Army specialist whom the FBI would eventually declare guilty, retained security clearance even while he was considered the prime suspect.

“Those who were viewed as initial ‘suspects’ in the case remained so until the facts and evidence determined they were not culpable,” the bureau’s assistant director in the Office of Congressional Affairs, Richard Powers, wrote in a response to their letter.

Nadler, Conyers and Scott urged Mueller to clarify the issues in the case.

“Important and lingering questions remain that are crucial for you to address, especially since there will never be a trial,” they wrote.

Ivins was the only suspect when he died.

The anthrax scare put Washington on alert in 2001, when envelopes containing the potentially lethal substance were sent to Sen. Patrick Leahy (D-Vt.) and then-Senate Majority Leader Tom Daschle (D-S.D.).

The scare in Washington was amplified by five anthrax-related deaths, including those of two U.S. Postal Service employees, and reports that similar letters were sent to NBC News anchor Tom Brokaw and the New York Post.

In his letter, Powers pointed out that the FBI and Department of Justice had held briefings for Congressional staff in August and September to answer questions about the case.

“It is our hope these briefings answered the majority of the questions outlined in your letter,” Powers wrote.

Nadler, however, remained skeptical about the evidence that exists against Ivins.

In particular, Nadler asked how much silica was found in the anthrax powder, the answer to which, he said, could help the FBI’s case — or throw it into serious question.

Silica occurs naturally in anthrax, so if less than 0.5 percent was found in the powder, Ivins is viable as the person who planted the anthrax, according to Nadler.

If the percentage is higher, however, it would indicate that the person who added the silica to the anthrax would have been specially trained in the procedure, which Ivins was not.

“Essentially, you’ve pinned the anthrax stuff on this fellow who conveniently died before they pinned it on him” without offering conclusive evidence of his guilt, Nadler said.

The Congressman asked Mueller about the percentage of silica at the hearing but did not get a satisfactory answer.

“He said, of course, he’ll have to get back to me.”

Any action by the committee now hinges on that answer, according to Nadler.

“If it’s less than 0.5 percent, I’m not sure we do anything,” he said. “[If it’s more] the question is, ‘Wait a minute. We have to redo the whole thing. Who is that person? Where did it come from?’ And stop pinning it on the dead guy.”