The man held responsible for the post-September 11 anthrax attacks may have had a secret accomplice, or been completely innocent of his alleged crimes, research into the FBI's investigation of the affair shows.
Dr Bruce Ivins, an army bio-defence expert, committed suicide in 2008 after learning that murder charges were about to be filed against him in connection with the terrorist campaign, in which five people were killed and 17 injured.
But an article published this week in the Journal of Bioterrorism & Biodefence highlights several inconsistencies in the forensic evidence against Ivins, raising speculation the FBI got the wrong man, and then closed their investigation after his death.
The report, co-authored by three scientists, says detectives failed to properly analyse the dried anthrax spores used in the attacks, which took place over several weeks following the September 11 bombings in 2001.
Analysis of the white powder, which was sent through the postal service to news organisations and politicians, showed it contains unexpected traces of tin. That suggests a high degree of manufacturing skill, contrary to official conclusions that the attacks were part of a relatively unsophisticated campaign carried out by Ivins alone.
Sceptics say Ivins made a convenient scapegoat for investigators.
There was no concrete evidence linking Ivins to the crime. A report published last year by the National Academy of Sciences concluded that the FBI did not have enough scientific evidence to produce a conviction, had the case gone to trial.