The F.B.I. agent who helped start the investigation that led to the resignation of David H. Petraeus as C.I.A. 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 on 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 e-mails 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 e-mails. It also ensnared Gen. John R. 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, in just his third year at the F.B.I., 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.
The F.B.I. investigation that led to the resignation of David H. Petraeus as C.I.A. director on Friday began with a complaint several months ago about “harassing” e-mails sent by Paula Broadwell, Mr. Petraeus’s biographer, to an unidentified third person, a government official briefed on the case said on Saturday.
When F.B.I. agents following up on the complaint began to examine Ms. Broadwell’s e-mails, they discovered exchanges between her and Mr. Petraeus that revealed that they were having an affair, said the official, who spoke of the investigation on the condition of anonymity.
The person who complained about harassing messages from Ms. Broadwell, according to the official, was not a family member or a government official. One Congressional official who was briefed on the matter on Friday said senior intelligence officials had explained that the F.B.I. investigation “started with two women.”
“It didn’t start with Petraeus, but in the course of the investigation they stumbled across him,” said the Congressional official, who said the intelligence officials had provided no other information about the two women or the focus of the inquiry. “We were stunned.”
Mr. Petraeus said in a statement that he was resigning after 14 months as head of the Central Intelligence Agency because he had shown “extremely poor judgment” in engaging in the affair.
The government official dismissed a range of media speculation that the F.B.I. inquiry might have focused on leaks of classified information to the press or even foreign spying. “People think that because it’s the C.I.A. director, it must involve bigger issues,” the official said. “Think of a small circle of people who know each other.”