Documents made public alleged that Ivins, who committed suicide last week, had sole custody of highly purified anthrax spores with "certain genetic mutations identical" to the poison used in the attacks. Investigators also said they had traced back to his lab the type of envelopes used to send the deadly spores through the mails.
Ivins killed himself last week as investigators closed in, and U.S. Attorney Jeffrey Taylor said, "We regret that we will not have the opportunity to present evidence to the jury."
The prosecutor's news conference capped a fast-paced series of events in which the government partially lifted its veil of secrecy in the case that followed closely after the airliner terror attacks of Sept. 11, 2001.
The newly released records depict the scientist as deeply troubled, bordering on desperation as he confronted the possibility of being charged.
"He said he was not going to face the death penalty, but instead had a plan to kill co-workers and other individuals who had wronged him," according to one affidavit.
The affidavits also said 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 unnamed co-workers.
He was also said to have received immunizations against anthrax and yellow fever in early September 2001, several weeks before the first anthrax-laced envelope was received in the mail.
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.
"We are confident that Dr. Ivins was the only person responsible for these attacks," Taylor told a news conference at the Justice Department.
Noting that Ivins would have been entitled to a presumption of innocence, Taylor nevertheless said prosecutors were confident "we could prove his guilt to a jury beyond a reasonable doubt."
Ivins' attorney, Paul Kemp, has repeatedly asserted his late client's innocence.
The events in Washington unfolded as a memorial service was held for Ivins at Fort Detrick, the secret government installation in Frederick, Md., where he worked. Reporters were barred.
More than 200 pages of documents were made public by the FBI, virtually all of them describing the government's attempts to link Ivins to the crimes.
"It is a very compelling case," said Rep. Chris Smith, R-N.J., who attended a briefing for lawmakers and staff.
The government material describes at length painstaking scientific efforts to trace the source of the anthrax that was 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 that 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 that matched came from a single batch, code-named RMR-1029, stored at Fort Detrick.
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 mailed to Sens. 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.
The documents disclosed that authorities searched Ivins' home on Nov. 2, 2007, taking 22 swabs of vacuum filters and radiators and seizing dozens of items. Among them were video cassettes, family photos, information about guns and a copy of "The Plague" by Albert Camus.
Investigators also reported seizing three cardboard boxes labeled "Paul Kemp ... attorney client privilege."
Ivins' cars and his safe deposit box also were searched as investigators closed in on the respected government scientist who had been troubled by mental health problems for years.
According to an affidavit filed by Charles B. Wickersham, a postal inspector, the scientist told an unnamed co-worker "that he had `incredible paranoid, delusional thoughts at times' and 'feared that he might not be able to control his behavior."'
A mental health worker who was involved in treating Ivins disclosed last week that she was so concerned about his behavior that she recently sought a court order to keep him away from her.
Allegations that Ivins sought to mislead investigators ran through the material made public.
One FBI document said Ivins "repeatedly named other researchers as possible mailers and claimed that the anthrax used in the attacks resembled that of another researcher" at the same facility.
The name of the other researcher was not disclosed.
Stephen A. Hatfill's career as a bioscientist was ruined after then-Attorney General John Ashcroft named him a "person of interest" in the probe. The government recently paid $6 million to settle a lawsuit by Hatfill, who worked in the same lab.
The documents made public painted a picture of Ivins seeking to mislead investigators beginning in 2002, when he allegedly submitted the wrong samples to FBI investigators.
It wasn't until more than two years later, in March 2005, that he was confronted with the alleged switch, according to U.S. Postal Inspector Thomas Dellafera, who added that Ivins insisted he had not sought to deceive.
The documents, which were expected to shed light on many of the mysteries surrounding the case, were released following an order from U.S. District Judge Royce Lamberth. Among them were more than a dozen search warrants issued as the government closed in on Ivins in an investigation into the terrifying mail poisonings a few weeks after the Sept. 11, 2001, terror attacks.
Lamberth ordered the release after consultation with Amy Jeffress, a national security prosecutor at the Department of Justice.
The investigation dates to 2001, when anthrax-laced mail turned up in two Senate offices as well as news media offices and elsewhere. At the time, the events were widely viewed as the work of terrorists, and delivery of mail was crippled when anthrax spores were discovered in mailing equipment that had processed the contaminated envelopes.
The FBI's investigation had dragged on for years, tarnishing the reputation of the agency in the process.