It’s been an enduring mystery: who sent that weaponized anthrax through the mail shortly after September 11, 2001? New information suggests that the FBI may be closer to solving the case: FBI Focusing on ‘About Four’ Suspects in 2001 Anthrax Attacks.

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. …

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!’”