DOVER, Fla. — The F.B.I. agent who spurred the investigation that led to the resignation of David H. Petraeus as C.I.A. director is a “hard-charging” veteran who helped investigate the foiled millennium terrorist plot in 1999, colleagues said on Wednesday.

The agent, Frederick W. Humphries II, 47, is also described by former colleagues as relentless in his pursuit of what he sees as wrongdoing, which appears to describe his role in the F.B.I. investigation involving Mr. Petraeus. Suspecting that the case involved serious security issues and was being stalled, possibly for political reasons — a suspicion his superiors say was unjustified — he took his concerns to Congressional Republicans.

“Fred is a passionate kind of guy,” one former colleague said. “He’s kind of an obsessive type. If he locked his teeth onto something, he’d be a bulldog.”

The question of how and why the F.B.I. opened the investigation that has had such momentous consequences has been central from the moment Mr. Petraeus stepped down Friday. The emerging portrait of the agent who initiated the inquiry is another step toward an answer.