By Evan Perez
FBI Director James Comey told a Senate hearing on Thursday the agency considers the investigation of the deadly Benghazi terror attack among its "highest priorities."
In response to questions from Sen. Kelly Ayotte, R-New Hampshire, Comey said the FBI still has "a lot of people working very, very hard on this. We are committed bringing to justice those responsible for the attack and the murder of our folks.
"These are often difficult cases to make, but as you've seen for our work, we never give up and we will never rest until we bring to justice the people responsible," he said.
Ambassador Christopher Stevens and three other Americans were killed in the armed assault on the U.S. diplomatic compound in eastern Libya in September 2012.
Comey appeared before the Homeland Security and Government Affairs Committee, his first congressional testimony since taking office in September.
He said that while terrorism remains the FBI's top priority, cyber-security could soon top that.
"We anticipate that in the future, resources devoted to cyber-based threats will equal or even eclipse the resources devoted to non-cyber based terrorist threats," he said.
Senators tried to draw Comey and his fellow panel members, Rand Beers, acting secretary of the Homeland Security Department, and Matthew Olsen, director of the National Counterterrorism Center, into taking sides on the ongoing partisan divide over the Guantanamo prison, which President Barack Obama has ordered closed.
Ayotte expressed frustration that the Obama administration's practice of interrogating terror suspects on Navy ships, as happened after the capture of alleged al Qaeda operative Abu Anas al Libi in Libya in October, was a poor substitute for Guantanamo.
She pressed Comey to say that longer periods of interrogation are better for law enforcement. He agreed.
Left unsaid: al Libi was brought to the United States after a week in part because he stopped cooperating and officials wanted to provide treatment for pre-existing Hepatitis, according to U.S. law enforcement officials.
Accused playing a role in the 1998 U.S. Embassy bombings in Kenya and Tanzania, he has pleaded not guilty to terrorism charges in federal court in New York.
Sen. Carl Levin, D-Michigan, rebutted the questioning from Ayotte and others by getting Comey to discuss the many terrorism trials that have been safely handled by civilian courts.
Beers and Comey said they knew of no particular danger posed by housing terrorists in maximum security federal prisons.
Comey tried to avoid the political fighting over Benghazi.
The FBI director said he didn't object to members of Congress interviewing U.S. survivors of the Benghazi attack, as requested by some lawmakers.
He said he was unable, in a public hearing, to discuss related charges filed under seal.
As first reported by CNN, the Justice Department has filed charges against suspects, including alleged Libyan militant Ahmed Abu Khattala, but hasn't yet made them public.
CNN has also reported that the United States has prepared plans to try to capture Khattala and other suspects, but that operation was been scuttled in the wake of the al Libi capture.
A U.S. law enforcement official has told CNN that a planned operation in Benghazi, a stronghold of hostile militant forces, was always viewed as more complicated than the al Libi operation in Tripoli, which is under greater Libyan government control.