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.

"Fort Detrick is run by the United States Army. It's the most secure biological warfare research center in the United States," a bioterrorism expert told FOX News.

Asked to comment on the likelihood that the anthrax originated at the facility, the expert said:

"It's not suprising, except that it would underscore that there was serious security deficiencies that existed at one time at Fort Detrick - the ability of researchers to smuggle out some type of very sophisticated anthrax weapon and in some quantity. And, nevertheless, it was possible."

In December 2001, an Army commander tried to dispel the possibility of a connection to Fort Detrick by taking the media on a rare tour of the base. The commander said the Army used only liquid anthrax, not powder, for its experiments.

"I would say that it does not come from our stocks, because we do not use that dry material," Maj. Gen. John Parker said. The letters that were mailed to the media and Sens. Tom Daschle and Patrick Leahy all contained powdered anthrax.

But in an e-mail obtained by FOX News, scientists at Fort Detrick openly discussed how the anthrax powder they were asked to analyze after the attacks was nearly identical to that made by one of their colleagues.

"Then he said he had to look at a lot of samples that the FBI had prepared ... to duplicate the letter material," the e-mail reads. "Then the bombshell. He said that the best duplication of the material was the stuff made by [name redacted]. He said that it was almost exactly the same ... his knees got shaky and he sputtered, 'But I told the General we didn't make spore powder!'"

Asked for comment, an Army spokeswoman referred all calls to the FBI. The FBI would not comment about the pool of suspects, but a spokeswoman said the investigation clearly remains a priority.