WASHINGTON (CBS News) -- Robert Mueller, who spent the last 12 years as FBI director -- the second longest serving director after J. Edgar Hoover -- is retiring soon. He turned the agency into one of the country's main terrorist fighting organizations.
One week on the job when 9/11 happened, there were no other major attacks on the homeland during his tenure. But in his final interview as director he said the threat of another attack is still there.
Mueller warned Thursday the al Qaeda threat which closed 19 U.S. diplomatic posts earlier this month, is still active.
"We're monitoring it very carefully and want to make certain that if it were postponed because of the actions we are taking, that we're alert to the possibility that it's back on for action," he said.
Al Qaeda in the Arabian Peninsula has tried to hit the U.S. before with the underwear bomb and the printer bombs. Asked if he expects the group will try to attack again, Mueller said, "I would expect that, down the road, that people would like to hit us."
Inside SIOC, the FBI's secure command center, Mueller explained terror has been his top worry since the beginning of his 12-year tenure. He was just a week into the job when 9/11 happened.
"In the wake of September 11, the one question that was always posed by the president, whether it be George Bush or President Obama, is, 'What is the FBI doing to prevent the next terrorist attack?'"
Mueller was forced to transform the FBI from a criminal investigative agency into a counterterrorism force. While there have been no major follow-up strikes, there have domestic attacks: Fort Hood in 2009 and the Boston Marathon bombings in April.
Despite criticism that the FBI missed some warning signs, Mueller said neither incident was preventable. A 2011 Russian warning about bombing suspect Tamerlan Tsarnaev turned up no terrorist connections.
"The agent who had that lead did a very thorough job in tracking down any information," Muller said. "He went to the college campus, did record checks, ultimately interviewed the parents and ultimately interviewed the individual."
Muller said he is satisfied the FBI did a thorough job.
Mueller also promised that people will be brought to justice for the attacks in Benghazi, which killed U.S. Ambassador Chris Stevens and three others. And Mueller expressed personal satisfaction with the conviction of Whitey Bulger, who was a mob boss in Boston back when Mueller was a prosecutor there.