DOVER, Fla. -- The FBI agent who helped start the investigation that led to the resignation of David Petraeus as CIA director is a "hard-charging" veteran counterterrorism investigator who used his command of French in investigating the foiled "millennium" terrorist plot in 1999, colleagues said Wednesday.
The agent, Frederick W. Humphries II, 47, took the initial complaint from Jill Kelley, the Tampa, Fla., hostess who was socially active in military circles there, about emails she found disturbing that accused her of inappropriately flirtatious behavior toward Mr. Petraeus.
The subsequent cyberstalking investigation uncovered an extramarital affair between Mr. Petraeus and Paula Broadwell, his biographer, who agents determined had sent the anonymous emails. It also ensnared Marine Gen. John Allen, who now commands troops in Afghanistan, after the investigation discovered that he had sent "inappropriate communication" to Ms. Kelley.
Colleagues and news reports described the role of Mr. Humphries, then in just his third year at the FBI, in building the case against Ahmed Ressam, who was detained as he tried to enter the United States from Canada in 1999 with a plan to set off a bomb at Los Angeles International Airport.
In May 2010, after he had moved to the Tampa field office, Mr. Humphries was attacked outside the MacDill Air Force Base gate by a disturbed knife-wielding man. He fatally shot the man, and the shooting was later ruled to be an appropriate use of force, according to bureau records and colleagues.
Two former law enforcement colleagues said Mr. Humphries was a solid agent with experience in counterterrorism, conservative political views and a reputation for aggressiveness.
"Fred is a passionate kind of guy," said one former colleague. "He's kind of an obsessive type. If he locked his teeth onto something, he'd be a bulldog."
That description would appear to fit his involvement in the current investigation.
Mr. Humphries passed along Ms. Kelley's complaint to the cybersquad in the Tampa field office, but he was not assigned to the case. He was later admonished by supervisors who thought he was trying to insert himself improperly into the investigation.
Convinced that the case was being stalled for political reasons, Mr. Humphries in late October contacted Republican Rep. Dave Reichert of Washington state, where the FBI agent had worked previously, to inform him of the case. Mr. Reichert put him in touch with House Majority Leader Eric Cantor, R-Va., who passed the message to FBI director Robert Mueller.
Lawrence Berger, general counsel for the Federal Law Enforcement Officers Association, who spoke with Mr. Humphries, said the agent only received the information from Ms. Kelley and never played a role in the investigation. Mr. Berger said Mr. Humphries and his wife had been "social friends with Ms. Kelley and her husband prior to the day she referred the matter to him. They always socialized and corresponded."
Mr. Berger took issue with news media reports that have said his client sent shirtless pictures of himself to Ms. Kelley. "That picture was sent years before Ms. Kelley contacted him about this, and it was sent as part of a larger context of what I would call social relations, in which the families would exchange numerous photos of each other," he said.
The photo was sent as a "joke" and was of Mr. Humphries "posing with a couple of dummies." He said the picture was not sexual in nature.
In regard to his client speaking with Mr. Cantor, Mr. Berger declined to address the issue, saying only that Mr. Humphries "had followed FBI protocols." He added: "No one tries to become a whistle-blower. Consistent with FBI policy, he referred it to the proper component."
A law enforcement official said disclosing a confidential investigation, even to members of Congress, could violate FBI rules. But the official said Mr. Humphries' conduct was under review, and that he had not been suspended or punished in any way.
Tuesday afternoon, a man standing in the driveway of Mr. Humphries' home said, in reply to a reporter's queries, that his first name was not Fred. The man then walked into the house, closed the front door and did not respond to the doorbell being rung several times.