Investigators believe anthrax used in a series of attacks in the US in 2001 was not of military grade as originally thought, a US newspaper reports.
At least five people died in anthrax attacks in 2001
The Washington Post paper says the FBI has widened its investigation into the source of the anthrax after finding it was of a more common variety.
"There is no significant signature in the powder that points to a domestic source," an expert told the paper.
Anthrax powder, sent by mail, killed five people in the US in October 2001.
At least 17 people were taken ill after coming into contact with the powder, which was sent in envelopes to US government offices and media organisations.
Coming weeks after the 11 September attacks, the anthrax mailings caused panic in the US.
Suspicion originally centred on scientists with access to US defence laboratories.
But no one was charged over the attacks, and according to the Washington Post, the pool of suspects has now been widened.
Officials quoted by the paper say the type of anthrax sent in the post bore none of the hallmarks associated with the "weaponised" form of the disease spores, used by the military.
The powder used in the attacks was reportedly of a common strain and had not been processed to make it more easily dispersible - and therefore, more lethal.
The paper quotes a report by an FBI scientist, Douglas J Beecher, who dismisses as a "widely-circulated misconception" the view that the anthrax spores were produced "using additives and sophisticated engineering supposedly akin to military weapons production".
According to another scientist interviewed by the Washington Post, although the anthrax used in the attacks was not military-grade, it was of a very high quality.
"Whoever did it was proud of their biology. They grew the spores, spun them down, cleaned up the debris. But there were no additives," the scientist is quoted as saying.