FBI field agents have returned to long-ignored leads in a continuing investigation of 2001 deaths related to anthrax (search).

Agents last week interviewed Ayaad Assaad (search), a former Army microbiologist for two hours, The Hartford Courant reported Sunday.

They asked detailed questions about his knowledge of drying anthrax into a fine powder similar to what was used in 2001, and took documents he offered them to show where he was when the first batch of letters was sent.

Assaad and his lawyer said the agents assured them on Tuesday that he is not a suspect.

FBI (search) agents interviewed Assaad once before, on Oct. 3, 2001, after an anonymous letter warned that he might be "planning to mount a biological attack," The Courant reported. But despite offers to provide more information, Assaad has not been re-interviewed since the first suspicious anthrax infection surfaced the following day.

FBI spokeswoman Debbie Weierman refused to comment on the significance of this week's Assaad interview. She also would not say how many other people have been questioned.

As of last October - the two-year anniversary of the anthrax attacks -- 30 FBI agents and 18 postal inspectors were working full time on the investigation, Weierman said.

Investigators believe that 22 people contracted anthrax after exposure to contaminated mail in the fall of 2001. Among the five who died was 94-year-old Ottilie Lundgren of Oxford.

In the first months of the investigation, FBI agents questioned dozens of current and former researchers from the U.S. Army Medical Research Institute of Infectious Diseases in Frederick, Md., where Assaad once worked. They repeatedly asked who at the institute had access to anthrax, and whether it would have been possible for someone to sneak the biological agent out of the facility.

More recently, federal authorities have shifted their focus to the complicated process of breaking down the gene sequence of the anthrax used in the attack to see if it can be linked to stocks from a specific laboratory.