WASHINGTON — For most of his career, he was a casting agent’s vision of a bench scientist: shy, eccentric, nerdy, soft-spoken. But sometime this spring, with the FBI closing in on him, Bruce Ivins’ life took a dark turn that frightened his closest friends.

In March, police officers summoned to a Frederick, Md., neighborhood found the 62-year-old microbiologist unconscious in his home. Four months later, he was admitted to a psychiatric clinic after threatening to “take out” co-workers at the Army research institute where he kept his lab. Then, a week ago, his therapist petitioned for a protection from Ivins. She described a man spiraling out of control, making “homicidal threats, actions, plans.”

His death Tuesday from an apparent drug overdose was followed by a revelation even more jarring to those who knew him: a report that Ivins had been implicated in the 2001 anthrax attacks, one of the FBI’s biggest unsolved mysteries and most baffling technical cases. Ivins was on the verge of being indicted in the case, according to officials familiar with the investigation, and took his life by swallowing a large quantity of prescription-strength acetaminophen.

Yet, slowly over the past two years, FBI investigators began to focus on Ivins under the theory that he had used his knowledge of anthrax bacteria to pull off the nation’s deadliest episode of biological terrorism.

As a researcher for the Army’s main lab for studying bioterrorism agents, Ivins had easy access to anthrax bacteria, including the specific strain used in the attacks on media outlets and congressional offices in fall 2001. His expertise eventually earned him a front-row seat for the FBI’s investigation, as he was called upon to help the bureau with its analysis of the powder used in the attacks.

Despite the allegations — and even after Ivins’ apparent plunge into mental illness — friends and colleagues say it is inconceivable that Ivins could have been a bioterrorist. Many contend that he was driven to depression and suicide because of months of hounding by federal investigators.