The FBI agent responsible for the downfall of two of the military’s most respected generals helped stop a terrorist from bombing Los Angeles International Airport and shot a man who attacked him with a knife at the gates of a military base. And he kicked off an investigation that not only upended Washington, it has many wondering if the FBI exceeded its authority.

Meet Frederick W. Humphries II — finally. Humphries, identified by The New York Times, is the mystery Florida-based FBI agent central to the ongoing scandal that brought down CIA Director David Petraeus and threatens the career of the Afghanistan war commander. At nearly every key moment in the tawdry sex scandal, Humphries has been there, lurking in the shadows, sometimes without his shirt on. No wonder colleagues interviewed by the Times described him as “obsessive.” Even before anyone knew who he was, someone set up a parody Twitter account for him, @shirtlessFBIguy.

In 1999, Humphries used his French language skills to interrogate a Francophone suspect. And that helped the Bureau find and stop Ahmed Ressam from bombing LAX airport in what would come to be known as the Millennium Plot, according to a Seattle Times piece. Described as “wiry [and] high-energy,” the former Army officer unraveled the cover story of a member of the Millennium Plot by calling bull on the operative’s fake Quebecois accent. Eleven years later, Humphries would shoot and kill a “disturbed knife-wielding man” who attacked him at the gates of MacDill Air Force Base in Tampa.

Humphries knew Tampa socialite Jill Kelley, an unofficial “ambassador” between Tampa and MacDill, home of U.S. Central Command, run in 2010 and 2011 by Petraeus and Gen. John Allen, now the commander of the Afghanistan war. When Kelley started receiving harassing e-mails this summer, Kelley asked her FBI friend Humphries to look into it. Humphries agreed, but soon found himself taken off the case, according to the Times. That would prove to be a fateful move.

The FBI has broad authorities over cyber-stalking investigations. “When something of this nature comes to our attention,” spokesman Paul Bresson tells Danger Room, “we work in close coordination with prosecutors to evaluate the facts and circumstances with respect to jurisdiction and potential violations of federal law.”

Not everyone is buying that the FBI would normally take up the case of a socialite receiving unwanted, nasty e-mails. “This is highly irregular. Highly, highly irregular. With a case of e-mail harassment, we’d normally say: we’re kind of busy, contact your local police,” a former federal prosecutor tells Danger Room. “You know that old cliche ‘let’s not make a federal case out of it?’ Well, in this case, it rings true.”

In any case, the feds did make a federal case out of it — just without Humphries. But Humphries didn’t let the case go. He sent shirtless pictures of himself to Kelley, something a lawyer for a law-enforcement guild who spoke with Humphries described to the Times as a “joke” that the national media have misunderstood. Still, his friends characterized him as “passionate” and “kind of an obsessive type.” It showed.

Humphries did not take kindly to being removed from a case he kickstarted. Evidently, he knew that the FBI expanded the case from cyber-harassment to one determining whether Paula Broadwell, Petraeus’ mistress who harassed Kelley, received classified information from Petraeus. Humphries was convinced there was a Bureau cover-up to protect Obama, and in late October went to Rep. Dave Reichert, a Washington state Republican with whom Reichert had worked previously. Reichert — who would not respond to Danger Room’s queries — took Humphries to Rep. Eric Cantor, the GOP majority leader, on October 27.

Cantor and his staff met with Humphries shortly after Reichert made the introduction. But they did not know what his motivations were. Nor could they judge Humphries’ credibility. Worse, they had no idea the FBI had Petraeus under investigation in the first place. After conferencing, they decided the prudent thing to do was to take the information from the investigation to FBI Director Robert Mueller’s office. They did so on October 31, around the same time that FBI agents interviewed Petraeus and reportedly told him he was not under suspicion of leaking classified information.

A week later, on November 6 — election day — Mueller informed James Clapper, the director of national intelligence and Petraeus’ boss, of the investigation. The House Judiciary Committee has written to Mueller to determine, among other things, why Mueller waited a week, and why he informed neither the relevant congressional oversight committees or the White House. (Mueller on Wednesday briefed the leaders of the House and Senate intelligence committees.) But Clapper essentially sealed Petraeus’ fate, urging him to deliver the resignation from the CIA that ultimately came on Friday.

There are questions about whether the FBI has exceeded its bounds in the case Humphries launched. While the FBI has wide latitude to investigate potential leaks of classified intelligence — the focus of the ongoing inquiry into Broadwell that brought Petraeus down — it is far less clear what authority the FBI had to give the Pentagon flirtatious emails between Allen and Kelley that came to agents’ attention in the course of that inquiry.

The Pentagon, whose inspector general is now investigating Allen, says there is no evidence Allen gave Kelley classified material or otherwise compromised national security. Under the Uniformed Code of Military Justice, adultery is a crime. But a Defense official on Tuesday told reporters that Allen denies cheating on his wife, and the emails contain some “flirtatious” exchanges between the two. Yet while the so-called “Plain Sight Doctrine” holds that investigators can pursue evidence of a crime that they encounter in an unrelated investigation, flirtation is not evidence of adultery.

While many of the facts of Allen’s case have yet to be determined, some legal experts wonder if the FBI was required to ignore the emails between Allen and Kelley.

“Whether the supposed basis for the investigation was cyber-harassment, disclosure of classified information, or the vulnerability of the CIA chief to blackmailing, it’s difficult to see how a military commander’s flirtatious emails are relevant,” says Rachel Levinson-Waldman, a lawyer who studies information sharing between national-security agencies at New York University’s Brennan Center for Justice. In such a case, the FBI is usually required to “minimize” — that is, ignore or destroy — information on unrelated parties that it inadvertently collected. In practice, though, Levinson-Waldman cautions, FBI officials have strong incentives to hold on to such material, for fear of jeopardizing potential future investigations.

The FBI, argues the Electronic Frontier Foundation’s Kurt Opsahl, appears to have engaged in “a series of stretches,” to get from investigating Broadwell to turning over Allen’s communications with Kelley to the Defense Department. “I don’t see how that email [traffic] is necessary or how there’s any kind of probable cause to believe there’s any link to the crimes the FBI was investigating,” Opsahl says.

In a statement released by his military lawyer late Wednesday, Allen vowed “to fully cooperate with the Inspector General Investigators” while his nomination to be NATO commander is officially on hold. There’s a possibility that Allen will be vindicated. But if he’s not, he has overzealous FBI investigators to thank — including Humphries, who started it all.

– additional reporting by Noah Shachtman