by Cather­ine Her­ridge and Ian McCaleb
FOXNews.com

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.