US Army scientist Bruce Ivins had sole custody of highly purified anthrax spores with "certain genetic mutations identical" to the poison that killed five people and rattled America in 2001, according to documents unsealed today in the government's investigation.
Investigators also reported tracing the type of envelopes used to send deadly spores through the post to the lab where Ivins worked.
The scientist, depicted in the newly released papers as deeply troubled, committed suicide last week as investigators were preparing to charge him with murder over the attacks.
The documents were released as the FBI held a private briefing for families of the victims of the episode, and officials said the agency was preparing to close the case.
More than 200 pages of documents were made public by the Federal Bureau of Investigation, virtually all of them describing the attempts to link Ivins to crimes that his lawyer has said he did not commit.
Senate Sergeant at Arms Terrance Gainer, who attended a briefing for congressional staff, said FBI agents had told the group there was no evidence that anyone else was involved.
According to one affidavit made public, Ivins submitted false anthrax samples to the FBI, was unable to give investigators "an adequate explanation for his late laboratory work hours around the time of" the attacks and sought to frame an unnamed co-worker.
He was also said to have been immunised against anthrax and yellow fever in early September 2001, several weeks before the first anthrax-laced envelope was received in the post.
The material describes at length painstaking scientific efforts to trace the source of the anthrax used in the attacks.
It says that in his lab, Ivins had custody of a flask of anthrax termed "the genetic parent" to the powder involved - a source investigators say was used to grow spores for the attacks on "at least two separate occasions".
Anthrax culled from the letters was quickly discovered to be the so-called Ames strain of bacteria, but with genetic mutations that made it distinct. Scientists developed more sophisticated tests for four of those mutations and concluded that all the samples which matched came from a single batch, codenamed RMR-1029, stored at a government laboratory where Ivins worked.
Ivins "has been the sole custodian of RMR-1029 since it was first grown in 1997", said one affidavit.
Powder from anthrax-laden letters sent to the New York Post and Tom Brokaw of NBC contained a bacterial contaminant not found in the anthrax-containing envelopes posted to Senators Patrick Leahy or Tom Daschle, the affidavit said.
Investigators concluded that "the contaminant must have been introduced during the production of the Post and Brokaw spores", the affidavit said.