Ivins, who committed suicide last week, submitted false anthrax samples to the FBI to throw investigators off his trail and was unable to provide "an adequate explanation for his late laboratory work hours" around the time of the attacks, according to documents that officials made public to support their case.
Investigators also said he sought to frame unnamed co-workers and had immunized himself against anthrax and yellow fever in early September 2001, several weeks before the first anthrax-laced envelope was received in the mail.
Ivins killed himself last week as investigators closed in, and U.S. Attorney Jeffrey Taylor said at a Justice Department news conference, "We regret that we will not have the opportunity to present evidence to the jury."
Ivins' attorney, Paul Kemp, has repeatedly asserted his late client's innocence, and Taylor conceded the evidence was largely if not wholly circumstantial.
The prosecutor's news conference capped a fast-paced series of events in which the government partially lifted its veil of secrecy in the investigation of the poisonings that followed closely after the airliner terror attacks of Sept. 11, 2001.
The newly released records depict Ivins as deeply troubled, increasingly so 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. In e-mails to colleagues, Ivins described a feeling of dual personalities, the material said.
Ivins had sole custody of highly purified anthrax spores with "certain genetic mutations identical" to the poison used in the attacks, according to an affidavit among a stack of documents the government released, all seemingly pointing to his guilt. Investigators also said they had traced back to his lab the type of envelopes used to send the deadly powder through the mails.
The FBI's investigation had dragged on for years, tarnishing the reputation of the agency in the process. Investigators had long focused on Steven J. Hatfill, whose career as a bioscientist was ruined after then-Attorney General John Ashcroft named him a "person of interest" in 2002. The government recently paid $6 million to settle a lawsuit by Hatfill, who worked in the same lab as Ivins.
Taylor said Wednesday that investigators concluded in 2005 that Hatfill couldn't have had access to a crucial flask of anthrax spores.
The prosecutor called the flask the murder weapon in the worst case of bioterror in the nation's history.
Authorities say that language Ivins used in an e-mail days before a second round of anthrax attacks was similar to the messages in anthrax-laced letters received soon after by Democratic Sens. Tom Daschle and Patrick Leahy.
CBS News correspondent Bob Orr reports that just before the deadly letters were mailed, Ivins sent a chilling e-mail to a friend warning: "Bin Laden terrorists for sure have anthrax and sarin gas...." and have "just decreed death to all Jews and all Americans."
The e-mail was strikingly similar, investigators say, to the language of the letters themselves, which read: "We have this anthrax... Death to America... Death to Israel."
Wednesday's documents were released as FBI Director Robert Mueller met privately with families of the victims of the attacks to lay out the evidence officials said the agency was preparing to close the case.
"He would periodically return in the evenings, presumably to check on the status of various experiments. Beginning in mid-August 2001, however, there was a noticeable spike in Dr. Ivins's evening access to the B3 hot suite" of the laboratory, according to the affidavit obtained by CBS News.
"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."
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 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 say Ivins strongly disagreed with Senators Daschle and Leahy's support for abortion rights, CBS' Orr reports.
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.
The documents 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 victims of the attacks had little in common.
Robert Stevens, 63, a photo editor at the Sun, a supermarket tabloid published in Boca Raton, Fla., was the first to die. Thomas Morris Jr. 55, and Joseph Curseen, 47, worked at a Washington-area postal facility that was a hub for sorting the capital's mail. Kathy Nguyen, 61, who had emigrated from Vietnam and lived in the Bronx, worked in a stock room at Manhattan Eye Ear & Throat Hospital. Ottilie Lundgren, 94, who lived in Oxford, Conn., was the last to die.