WASHINGTON —  The FBI has narrowed its focus to “about four” suspects in the 6 1/2-year investigation of the deadly anthrax attacks of 2001, and at least three of those suspects are linked to the Army’s bioweapons research facility at Fort Detrick in Maryland, FOX News has learned.

Among the pool of suspects are three scientists — a former deputy commander, a leading anthrax scientist and a microbiologist — linked to the research facility, known as USAMRIID.

The FBI has collected writing samples from the three scientists in an effort to match them to the writer of anthrax-laced letters that were mailed to two U.S. senators and at least two news outlets in the fall of 2001, a law enforcement source confirmed.

The anthrax attacks began shortly after the Sept. 11, 2001, terror attacks, further alarming a nation already reeling from the deaths of 3,000 Americans. Five people were killed and more than a dozen others were infected by the deadly spores in the fall of 2001.

A leading theory is that the anthrax was stolen from Fort Detrick and then sealed inside the letters. A law enforcement source said the FBI is essentially engaged in a process of elimination.

Much of the early public focus fell on a Fort Detrick scientist named Steven Hatfill, who is suing federal authorities for identifying him as a person of interest. Now the FBI is focusing on other scientists at the facility.

FOXNews.com - FBI Focusing on About Four Suspects in 2001 Anthrax Attacks

Is the CIA’s “Mighty Wurlitzer” Playing Geraldo Rivera?On Fox and Friends, Geraldo declared Times Square is a “magnet for these crazy people, for anarchists… that’s what I think this is, this bomber isn’t al-Qaeda, isn’t anything like that, he is more like — it’s a man — he’s more like those 9/11 was an inside job kind of guys, they… they protest things in a violent way, but in a violent way almost like the eco-terrorist where they don’t intend the inflict casualties.”

Alex Jones’ Infowars: There’s a war on for your mind!