Federal authorities released hundreds of pages of documents yesterday in an effort to show that they could prove beyond a reasonable doubt that Bruce Edwards Ivins, the Army scientist who killed himself last week, was the sole person responsible for the deadly 2001 anthrax attacks.

The investigators explained how they traced the anthrax used in the attacks back to Ivins' lab at Fort Detrick in Frederick, how Ivins allegedly stymied their investigation, and how what they called a history of mental illness and obsessive behavior helped them build a case that is circumstantial but, they said, irrefutable.

"The painstaking investigation led us to the conclusion that Dr. Bruce E. Ivins was responsible for the death, sickness and fear brought to our country by the 2001 anthrax mailings and it appears, based on the evidence, that he was acting alone," said Joseph Persichini, assistant director of the FBI's Washington field office.

Officials said they were taking the "extraordinary" step of releasing search warrants, affidavits and other documents because of keen public interest in the massive investigation into one of the most perplexing cases of domestic terrorism. But the disclosure also underscores the deep skepticism facing the FBI and the Justice Department, which were unable to solve the mystery for years.

Five people were killed and 17 injured from anthrax that was sent in letters to Congress and news organizations in September and October 2001. The lengthy federal probe into the attacks hit several dead-ends and led to a $5.8 million payment to a man wrongly identified as a person of interest, Steven J. Hatfill, who also worked at Fort Detrick.

A memorial service was held for Ivins yesterday at Fort Detrick, where the scientist had labored for decades to develop vaccines for anthrax. The service was attended by hundreds of soldiers, scientists and family members, according to Ivins' lawyers, who released a statement saying that no concrete evidence had been presented against their client.

"The government's press conference was an orchestrated dance of carefully worded statements, heaps of innuendo and a staggering lack of real evidence - all contorted to create the illusion of guilt by Dr. Ivins," said the lawyers, Paul F. Kemp and Thomas M. DeGonia. They have repeatedly said he was innocent.

But Ivins clearly knew investigators were closing in. His house had been under surveillance for a year, federal agents followed him wherever he went and he had been interviewed by investigators several times. Ivins had a history of mental illness, but the pressure led to a further decline in his condition, colleagues have said, to the point that he was hospitalized.

On July 9, Ivins was attending a group therapy session in Frederick. According to an affidavit released yesterday, he told the group he was a suspect in the anthrax investigation and that he was angry at the government and the investigators.

"He said he was not going to face the death penalty," the affidavit said, "but instead had a plan to kill co-workers and other individuals who had wronged him. He said he had a bulletproof vest, and a list of co-workers, and added that he was going to obtain a Glock firearm from his son within the next day, because federal agents are watching him and he could not obtain a weapon on his own."

He was admitted to a psychiatric wing of Frederick Memorial Hospital the next day. Shortly after he was released, on July 24, he took the overdose of Tylenol and codeine that killed him.

Yesterday, federal lawyers asked a judge in Washington to unseal search warrants and affidavits that spelled out their case, and FBI Director Robert S. Mueller III met with families of the anthrax victims and lawmakers to lay out the evidence against Ivins. One congressman who was briefed said that the presentation was "compelling" but that he was not convinced Ivins acted alone.

According to documents released, investigators traced the anthrax to Ivins' lab in 2005 using newly developed genetic mapping techniques. They concluded that all the anthrax used in the mailings that led to fatalities came from a single flask of anthrax spores known as RMR-1029.

That flask was "created and solely maintained by Dr. Ivins," said Jeffrey Taylor, U.S. attorney for the District of Columbia. "No one received material from that flask without going through Dr. Ivins. We thoroughly investigated every other person who could have had access to the flask, and we were able to rule out all but Dr. Ivins."

The time needed for that investigation explains why Ivins was not identified as the principal suspect until two years later, in 2007, Taylor said. There was "a tremendous amount of additional investigation that needs to take place to identify the universe of individuals who have access to that flask," he said. "We're talking about a large number of individuals, over 100, who potentially had access to this substance."

Ivins' off-hours time in the lab also raised suspicions. Just before the first set of anthrax letters was mailed, on Sept. 18, 2001, Ivins spent long hours at night in the lab containing the anthrax. And then again, before the second set of letters was mailed, on Oct. 9, he spent many night hours in the lab. He was alone during that time, officials said, suggesting he had the time to convert the anthrax into the deadly form that turned up in the letters.

When investigators asked Ivins about those extended hours, he offered "no legitimate reason," according to an affidavit, except that "home was not good" and he went to the lab "to escape."

Ivins "engaged in behavior and made a number of statements that suggest consciousness of guilt," Taylor said, including discarding a book on DNA coding while under round-the-clock surveillance. And in 2002, when investigators asked Ivins to provide a set of anthrax samples from the RMR-1029 flask, he provided a different set instead.

The correct sample was not provided until 2004.