Frederick W. Humphries II, the F.B.I. agent who spurred the investigation into Paula Broadwell, is a hard-charging, passionate agent who has played a leading role in foiling numerous terrorist plots, friends and colleagues say.

The New York Times reports that Humphries first became involved in the case when Jill Kelley, a friend of he and his wife’s, came to Humphries complaining about harassing emails from an unknown sender.

Colleagues at the F.B.I. confirm that once Kelley’s complaint reached Humphries’ desk, the veteran agent doggedly pursued it.

Fearing that his report had been stalled for political reasons, Humphries reached out to Republican Representative Dave Reichert, who Humphries knew from when he was working with the F.B.I. in Seattle. Reichert then put Humphries in touch with House majority leader Eric Cantor, who passed the information along to F.B.I. director Robert S. Mueller III.

The subsequent cyber-stalking investigation uncovered evidence of an affair between General David Petraeus and his biographer Paula Broadwell. It also discovered a sexually explicit online relationship between Jill Kelley and General John R. Allen, the commander of NATO forces in Afghanistan.

In prior work, Humphries had a reputation for being an aggressive agent with expertise in counterterrorism. He’s best remembered by law enforcement as the man who brought down Ahmed Ressam, the so-called “Millenium bomber,” who tried to detonate a bomb at Los Angeles International Airport under orders from Osama Bin Laden in 1999.

According to The Seattle Times, customs agents had detained Ressam after they felt he suspiciously entered the country from a Canadian ferry. White powder, mysterious chemicals, and timing devices were all found on Ressam, but little was known about him.

Humphries received a call about the case when the agent handling the complaint was unable to communicate with Ressam, who only spoke French. A French speaker, Humphries was called in and identified that Ressam’s accent wasn’t French-Canadian, which is where Ressam told law enforcement he was from.

Humphries understood the accent to be Algerian. The case unraveled from there as agents began building a profile of Ressam’s identity.

In 2007, according to ABC News, Humphries was called into testify against University of South Florida students Youssef Megahed and Ahmed Mohamed, who police found in possession of chemicals, gasoline, and PVC products during a traffic stop.  They were arrested for attempting to transport explosives.

In 2010, according to The New York Times, Humphries fatally shot a knife-wielding man in front of MacDill Air Force Base in Tampa, the location volunteer military social liaison Jill Kelley worked out.

Humphries admits to sending topless pictures of himself to Kelley’s cell phone, but says the photos were sent years before the current incident, and involved a prank between the two families.