WASHINGTON - The FBI sought to close the book on its long, frustrating hunt for the killer behind the 2001 anthrax letters Friday, formally ending its investigation and concluding a mentally unhinged scientist was responsible for killing five people and unnerving Americans nationwide.

After years of false leads, no arrests and public criticism, the FBI and Justice Department said Dr. Bruce Ivins, a government researcher, acted alone.

Ivins killed himself in 2008 as prosecutors prepared to indict him for the attacks. He had denied involvement, and his family and some friends have continued to insist he was innocent.

Investigators had tried earlier to build a case against biowarfare expert Steven Hatfill, who had worked for a time in the same military lab as Ivins, but ultimately turned away from that theory and had to pay him a multimillion-dollar settlement.

Many details of the case have already been disclosed, but newly released FBI documents paint a fuller portrait of Ivins as a troubled doctor whose life's work was teetering toward failure at the time the letters laced with anthrax were sent. As the U.S. responded to the mailings, that work was given new importance by the government, and he was even honored for his efforts on anthrax.

The documents also describe what investigators say was Ivins' bizarre, decades-long obsession with a sorority. The anthrax letters were dropped in a mailbox near the sorority's office in Princeton, N.J.

The letters were sent to lawmakers and news organizations as the nation reeled in the aftermath of the Sept. 11, 2001, terrorist attacks. Postal facilities, U.S. Capitol buildings and private offices were shut for inspection and cleaning by workers in hazardous materials "space suits" from Florida to Washington to New York and beyond.

In closing the case, officials also released reams of evidence, and a 92-page summary of their findings.

To the FBI's critics, the mountain of new documents could not paper over what they say are glaring holes in the case.

"The evidence the FBI produced would not, I think, stand up in court," said Rep. Rush Holt, a Democrat whose New Jersey district includes the Princeton mailbox used in the attacks. "But because their prime suspect is dead and they're not going to court, they seem satisfied with barely a circumstantial case."

Ivins' lawyer Paul Kemp said he saw nothing new in the findings. "All they have confirmed is that they suspected him belatedly after finding out he had psychological problems," he said. "Sadly they substitute that for proof."

Authorities say Ivins' death capped a years-long cat-and-mouse game with investigators, in which he repeatedly offered to help the FBI catch the killer, cast suspicion on his colleagues and tried numerous forms of subterfuge.

After the attacks, the FBI relied heavily on Ivins' help, according to the documents, and the scientist offered agents his notebooks, his office and his e-mail. FBI agents found him easygoing and funny. He had kittens on his computer screen saver and gave one FBI agent a jar of syrup for a gift.

He passed a polygraph in connection with the probe in 2002, but investigators learned years later that he had been prescribed psychotropic medications at the time, and examiners who reassessed the results concluded he exhibited classic signs of the use of countermeasures to pass the test.