SEPTEMBER 26, 2001 : (ANONYMOUS LETTER IS POSTMARKED ON THIS DATE ALLEGING THAT RESEARCHER ASSAAD WAS PLANNING TO MOUNT A BIOLOGICAL ATTACK) looked into the allegation [an anonymous letter the FBI had received, purportedly from an Assaad co-worker, warning that Assaad might be planning to mount a biological attack ] ...The letter was dated Sept. 26, 2001, eight days after the first batch of anthrax-laced letters went out but before their effects became known. -- "FBI anthrax probe revisits former Detrick researcher," by DAVID DISHNEAU, The Associated Press, 5/16/2004, 6:32 p.m. ET
Other good clues.
The chief difference was that a stretch of DNA was flipped head to tail in some bacteria in the attack strain, but not in any other samples.
Further, the attack strain contained bacteria with both the flipped and the unflipped DNA, showing that it was a mixture of two strains, which analysts later found reflected a mix of origins — 85 percent from the Dugway Proving Ground of the Army in Utah and 15 percent added at Fort Detrick, according to one person close to the investigation.