Aug. 6 (Bloomberg) -- U.S. law enforcement officials said government scientist Bruce E. Ivins was the only person behind the deadly 2001 anthrax attacks, pointing to newly unsealed evidence.

``We believe that based on the evidence we had collected, we could prove his guilt to a jury beyond a reasonable doubt,'' Jeffrey Taylor, U.S. attorney for the District of Columbia, said at a news conference. ``We are confident that Dr. Ivins was the only person responsible for these attacks.'' Taylor said the investigation will be closed.

Five people were killed and 17 others were infected when anthrax-laced letters were mailed to news organizations and members of Congress during the weeks after the Sept. 11 attacks.

Ivins, 62, was a scientist at the U.S. biodefense laboratory at Fort Detrick, Maryland, and helped investigators analyze anthrax samples used in the attacks. He died July 29 from an overdose of prescription Tylenol in what police are treating as a suicide. He had recently learned he was about to be charged in the attacks.

Ivins's lawyer, Paul F. Kemp, reasserted his client's innocence, saying the news conference was ``an orchestrated dance of carefully worded statements, heaps of innuendo and a staggering lack of real evidence.'' In a statement, the lawyer said Ivins ``worked for more than 30 years to defend his nation and its soldiers against the terrible effects of anthrax.''

Hatfill Settlement

The FBI's investigation previously came under criticism, and in June the agency agreed to pay a settlement valued at $5.8 million to another government scientist, Steven Hatfill, who said he was unfairly identified as a ``person of interest'' in the case.

Taylor cited evidence released by a court earlier today that indicates Ivins tried to mislead investigators and couldn't explain his late-night hours in the lab before the deadly letters were mailed. He said the materials were released because of the ``extraordinary and justified public interest in the case.''

Government officials briefed victims of the attack or their survivors earlier today.

The documents describe Ivins as feeling pressure to justify his work on an anthrax vaccine, which had come under criticism for possibly contributing to illness among Gulf War veterans. ``He's very concerned that this vaccination program he's been working on may come to an end,'' Taylor said.

`False Samples'

The newly released documents include a postal inspector's statement that Ivins ``is believed to have submitted false samples of anthrax from this lab to the FBI for forensic analysis in order to mislead investigators.''

An analysis of the electronic security system that controlled access to the tightly guarded facility showed that Ivins spent more than 30 hours at night in his lab during the month of September 2001, when the attacks began, court papers said.

He spent more than 15 hours at night in the lab in October, 2001. In all of 2000, Ivins had never spent more than five hours in the lab at night each month, the papers said.

The spike in his evening hours began in mid-August, almost a month before the Sept. 11 attacks, investigators said.

The anthrax attacks disrupted the U.S. mail and forced the shutdown of Senate offices after letters were delivered to Senators Tom Daschle and Patrick Leahy. Television news anchors Tom Brokaw and Dan Rather were also targeted.

Daschle, a South Dakota Democrat who has since left the Senate, said Aug. 3 on ``Fox News Sunday'' that he had ``real concerns about the quality of the investigation'' from the beginning.

Social Worker

A social worker who had treated Ivins in group therapy, Jean C. Duley, testified in July that he had shown violent tendencies, threatening her and talking about killing his colleagues. Duley obtained a restraining order against Ivins after his release from a four-week stay at a Maryland psychiatric hospital.

During a July 9 group therapy session, Ivins told his social worker and fellow group members that, rather than face the death penalty for the anthrax killings, he ``had a plan to kill co-workers and other individuals who had wronged him,'' said an affidavit filed by law enforcement officials.

Law enforcement officials suggested that Ivins's ``mental- health issues'' were caused by ``significant stress in both his home and work life.'' Ivins, who complained in e-mails of feeling paranoia, was treated by a psychiatrist in 2000, the court papers said.

Even with the treatment, ``the counseling, the depression episodes still come and go,'' Ivins wrote in a June, 27, 2000, e-mail. ``What is REALLY scary is the paranoia,'' he said, adding that he felt the return of ``feelings of isolation -- and desolation -- that I went through before college.''

To contact the reporters on this story: Greg Stohr in Washington at gstohr@bloomberg.net; James Rowley in Washington at jarowley@bloomberg.net

Last Updated: August 6, 2008 17:54 EDT