The researcher, Steven Hatfill, 48, had previously been questioned in the investigation of last fall's anthrax attacks and had his apartment searched in June.

CNN: FBI searches apartment in anthrax probe

Early in the investigation, then-Attorney General John Ashcroft publicly identified former civilian anthrax researcher Steven Hatfill as a "person of interest" in the investigation.

CNN: Therapist: Ivins described plot to kill colleagues

Wednesday's announcement comes six years to the day after then-Attorney General John Ashcroft publicly declared another Fort Detrick scientist, Steven Hatfill, a "person of interest" in the attacks.

CNN: U.S. officials declare researcher is anthrax killer

Early in the investigation, Attorney General John Ashcroft publicly identified a "person of interest" in the anthrax case -- Steven Hatfill, a former civilian researcher on anthrax.

CNN: Anthrax suspect, scientist, kills self as FBI closes in

Tom Daschle, who was Senate majority leader at the time, said he is suspicious of the case against researcher Bruce Ivins because of the government "bungling" of Steven Hatfill's case.

CNN: Intended anthrax target has doubts about probe

The sources said the FBI received a tip that former U.S. Army biomedical researcher Steven Hatfill may have disposed of certain laboratory equipment in the pond in Gambrill State Park.

CNN: Anthrax agents hunt lab equipment in pond

Ashcroft said the investigation has become more intense in recent weeks, but he refused to call Steven Hatfill -- a former federal scientist whose home has been searched as part of the investigation -- a suspect in the case.

CNN: Ashcroft: No charges yet in anthrax probe