FBI Special Agent Frederick Humphries II played a key role in stopping a terrorist attack aimed at blowing up Los Angeles International Airport just as the year 2000 dawned. He fatally shot a man who threatened him and other law enforcement officers with a knife during an altercation at Tampa's MacDill Air Force Base in 2010.

And now Humphries finds himself in the middle of the widening scandal that has resulted in CIA Director David Petraeus' resignation.

Humphries, 47, was the agent who initially saw the emails the FBI said Petraeus' biographer and mistress, Paula Broadwell, sent to Tampa socialite Jill Kelley, a woman she apparently saw as a rival for Petraeus' affections. She also allegedly sent emails to Gen. John Allen, Kelley's friend and the top US commander in Afghanistan.

Concerned about the emails, Kelley contacted Humphries in June. The two had met at a 2011 FBI Citizens Academy, a program aimed at teaching the public and journalists about what the agency does and how it operates. Kelley was in the class, which Humphries lectured one night about terrorism, according to Natalie Shepherd, a Tampa TV reporter who was there.

Humphries, a former Army captain who worked in military intelligence, thought the emails raised serious concerns because the anonymous author knew the comings and goings of Allen and Petraeus, a former general who had preceded Allen in Afghanistan. His report back to the FBI started the investigation that led to Broadwell and uncovered her affair with Petraeus.

The FBI is reviewing Humphries' later conduct in this case, a federal law enforcement official said Wednesday. Specifically, the bureau is reviewing a telephone call he made in late October to Rep. Dave Reichert, R-Wash., to voice concern that the bureau was not aggressively pursuing a possible national security breach. Reichert arranged to convey the information to House Majority Leader Eric Cantor of Virginia, who checked with the FBI at that time. Cantor was assured the bureau was on top of any possible vulnerability.

Lawrence Berger, the general counsel for the Federal Law Enforcement Officers Association, said in an interview Wednesday that his client, Humphries, did nothing wrong and should not be disciplined. "He's committed no misconduct," Berger said and predicted he would be cleared of any misconduct.

Humphries joined the FBI in 1996 and first came to prominence in 1999 after an Algerian man was arrested by US Customs agents as he tried to enter Washington state from Canada by ferry. Ahmed Ressam had white powder, chemicals and homemade timing devices in the trunk of his car.

Ressam claimed to be a French-speaker from Quebec. According to a 2002 Seattle Times story, Humphries, then the only French-speaker assigned to the FBI's Seattle office, was asked to question him. As soon as Humphries heard Ressam's accent and phrasing, he knew he wasn't from Quebec and told his colleagues to arrest him, the story said. It was later learned Ressam was part of an al-Qaida plot to blow up the Los Angeles airport on New Year's Eve 1999.

By 2010, Humphries had been assigned to the FBI's Tampa office and was its liaison to MacDill, home to the military's Central Command.

On May 19, 2010, 61-year-old Army veteran Ronald J. Bullock, who was camping at the base, got into an altercation with base security.

Officials said Bullock sped off on a motorcycle, but was stopped by other security officers and Humphries as he tried to exit a gate. They said he got off the motorcycle and came at Humphries and the officers while brandishing a knife. Humphries fired, killing Bullock. The shooting was later ruled justified.