"We are confident that Dr. Ivins was the only person responsible for these attacks," Jeffrey Taylor, the U.S. Attorney for the District of Columbia, told a news conference at the Justice Department.
The Justice Department's claim was backed by dozens of documents unsealed earlier in the day that pointed to the dead scientist's guilt.
The papers showed 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. Law enforcement officials also said they had traced the type of envelopes used to send the deadly spores through the mail back to his lab.
Ivins also could not give investigators "an adequate explanation for his late laboratory work hours around the time of" the attacks, and he apparently sought to mislead investigators on the case, according to an affidavit filed by one government investigator, the Associated Press reported.
"We are now beginning the process of concluding this investigation. Once this process is complete, we will formally close the case," Taylor said.
The letters also sickened 17 people, severely disrupted mail service and spread fear of further biological attacks among Americans reeling from the Sept. 11, 2001, airliner attacks.
The documents said that just before the anthrax attacks, Ivins sent an e-mail that included language similar to that in the anthrax letters warning that Osama "bin Laden terrorists" had anthrax and deadly sarin gas.
Referring to Ivins, Taylor told reporters, "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.
The prosecutor's Wednesday afternoon news conference capped a fast-paced series of events in which the government partially lifted its veil of secrecy in the case. 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," one affidavit read.
The affidavits also said Ivins submitted false anthrax samples to the FBI and sought to frame unnamed co-workers.
The documents were released as the FBI held a private briefing for families of the victims, and officials said the agency was preparing to close the case.
A federal judge unsealed documents related to the attacks on Wednesday, as the Justice Department prepared to declare, despite lingering skepticism, that the case had been solved, according to media reports.
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 from the event.
The documents disclose 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 videotapes, family photos, information about guns and a copy of "The Plague" by Albert Camus.
They also seized three 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,'" according to the AP.
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.
Robert M. Blitzer, who used to direct the FBI's section on domestic terrorism, scoffed at criticism of the investigatory methods used in the anthrax case and called them a necessary part of tracking down a killer.
"You do the best you can, and it's not always pretty," he told the New York Times. "A lot of times you interview folks over and over again, and you know they're lying and you've got to figure out why. It's a tough business. Here, you have a bunch of people dead and several diminished and you're charged with solving the crime. You try not to step on peoples' toes, but sometimes it happens."
Chief Judge Royce C. Lamberth of the Federal District Court in Washington, D.C., unsealed the documents after DOJ officials briefed relatives of some of the five people killed and 17 injured in the attacks.
Attorney General Michael B. Mukasey said Tuesday that the authorities had a "legal and moral obligation" to speak with the victims' families before releasing the information to the public, according to the Times.