Seven years after anthrax-laced letters terrorized an already frightened nation, a new suspect suddenly emerged in the public eye this week. But he was gone just as quickly, committing suicide before he could be charged with murder and adding another dramatic twist to the bizarre episode.

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The government's working theory — that brilliant but troubled Army scientist Bruce E. Ivins released the anthrax to test his cure for the toxin — answers some of the questions, perhaps, but many details remain unclear.

"I think the FBI owes us a complete accounting of their investigation and ought to be able to tell us at some point, how we're going to bring this to closure," said former Senate Democratic leader Tom Daschle of South Dakota, whose office received a letter containing the deadly white powder in 2001. "It's been seven years, there's a lot of unanswered questions and I think the American people deserve to know more than they do today."

Ivins' unexpected emergence as the top — and perhaps only — suspect in the anthrax attacks follows on the heels of the government's exoneration of another Army scientist in the case. Last month, the Justice Department cleared Ivins' colleague, Steven Hatfill, who had been wrongly suspected in the case, and paid him $5.8 million.

Responding to reports about Ivins on Friday, the Justice Department said only that "substantial progress has been made in the investigation" but said it may be able to release more information about the case soon. The department is expected to decide within days whether to close the "Amerithrax" investigation now that its main target is dead.

"We need to know exactly how Mr. Ivins was involved, if he was involved, how this relates to the case and information that so far has been withheld from the American people ought to be provided," Daschle said. "And I think it should be soon."

Bennet Bolton, a friend of the first anthrax victim — Robert Stevens — was suspicious about Ivins suicide and whether the government will disclose what happened.

"I don't think this guy was involved," Bolton said, questioning what led investigators from his dead friend — a tabloid photo editor in southern Florida — to the scientist at the Army's biological warfare labs at Fort Detrick, Md.

"What is the connection?" Bolton asked. "What did he do or not do?"

For 35 years, Ivins was one of the government's leading scientists researching vaccines and cures for anthrax exposure. But he also had a long history of homicidal threats, according to papers filed last week in local court by a social worker.

The letters containing anthrax powder were sent while the nation was still traumatized by the Sept. 11, 2001, attacks and turned up at congressional offices, newsrooms and elsewhere, leaving a deadly trail through post offices on the way. The powder killed five and sent numerous victims to hospitals and caused near panic in many locations.

Workers in protective garb that made them look like space men decontaminated U.S. Capitol buildings after anthrax letters were discovered there. Major postal substations were closed for years. Newsrooms were checked all over after anthrax letters were mailed to offices in Florida and New York.

Several U.S. officials said prosecutors had been focusing on the 62-year-old Ivins and planned to seek an indictment and the death penalty. Authorities were investigating whether Ivins, who had complained about the limits of testing anthrax drugs on animals, had released the toxin to test the treatment on humans.

The officials all discussed the continuing investigation on condition of anonymity because they were not authorized to speak to the media.

Ivins' attorney asserted the scientist's innocence and said he had cooperated with investigators for more than a year.

"We are saddened by his death, and disappointed that we will not have the opportunity to defend his good name and reputation in a court of law," said Paul F. Kemp.

Ivins died Tuesday at Frederick Memorial Hospital in Maryland. Relatives told The Associated Press that he killed himself. Kemp said his client's death was the result of the government's "relentless pressure of accusation and innuendo."

For more than a decade, Ivins had worked to develop an anthrax vaccine that was effective even in cases where different strains of anthrax were mixed — a situation that made vaccines ineffective — according to federal documents reviewed by The Associated Press.

In 2003, he shared the Decoration for Exceptional Civilian Service for his work on the anthrax vaccine. The award is the highest honor given to Defense Department civilian employees.

Ivins conducted numerous anthrax studies, including one that complained about the limited supply of monkeys available for testing. The study also said animal testing couldn't accurately show how humans would respond to anthrax treatment.

The Fort Detrick laboratory and its specialized scientists for years have been at the center of the FBI's investigation of the anthrax mailings. In late June, the government exonerated Hatfill, whose name has for years had been associated with the attacks. Then-Attorney General John Ashcroft called him a "person of interest" in 2002.

Investigators also had noticed Ivins' unusual behavior at Fort Detrick in the six months following the anthrax mailings. He conducted unauthorized testing for anthrax spores outside containment areas at the infectious disease research unit where he worked, according to an internal report. But the focus stayed on Hatfill.

Ivins' friends, colleagues and court documents paint a picture of a flourishing scientist with an emotionally unstable side. Maryland court documents show he recently received psychiatric treatment and was ordered to stay away from a woman he was accused of stalking and threatening to kill.

Social worker Jean C. Duley filed handwritten court documents last week saying she was preparing to testify before a grand jury. She said Ivins would be charged with five capital murders.

"Client has a history dating to his graduate days of homicidal threats, plans and actions towards therapists," Duley said, adding that his psychiatrist had described him as homicidal and sociopathic.

Authorities have been watching Ivins for some time. His brother, Tom Ivins, said federal agents questioned the scientist about a year and a half ago. Neighbors said FBI agents in cars with tinted windows conducted surveillance on his home. A colleague, Henry S. Heine, said that over the past year, he and others on their team had testified before a federal grand jury in Washington that has been investigating the anthrax mailings.