Federal prosecutors investigating the 2001 anthrax attacks were planning to indict and seek the death penalty against a top Army microbiologist suspected of releasing the deadly toxin to test anthrax drugs he was developing. The scientist committed suicide this week.

The scientist, Bruce E. Ivins, worked for the past 18 years at the government’s biodefense labs at Fort Detrick, Md. For more than a decade, he worked to develop an anthrax vaccine that was effective even in cases where different strains of anthrax were mixed, which made vaccines ineffective, according to federal documents reviewed by the AP.

In his research, he complained about the limitations of testing anthrax drugs on animals.

Several U.S. officials, all of whom discussed the ongoing investigation on condition of anonymity because they were not authorized to speak to the media, said prosecutors were closing in on Ivins, 62, for the attacks that killed five people, crippled the postal system and traumatized a nation still reeling from the Sept. 11 attacks.

Authorities had been investigating whether the anthrax was released to test new drugs. They were planning an indictment that would have sought the death penalty for the attacks, officials said.

The Justice Department has not yet decided whether to close the investigation, officials said, meaning authorities are still not certain whether Ivins acted alone or had help. One official close to the case said that decision was expected within days. If the case is closed soon, one official said, that will indicate that Ivins was the lone suspect.

Ivins’ attorney said the scientist 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,” attorney Paul F. Kemp said. “We assert his innocence in these killings, and would have established that at trial.”

Kemp said that Ivins’ death was the result of the government’s “relentless pressure of accusation and innuendo”

Ivins died Tuesday at Frederick Memorial Hospital in Maryland. Tom Ivins, a brother of the scientist, told The Associated Press that his other brother, Charles, had told him that Bruce committed suicide and Tylenol might have been involved. The Los Angeles Times, which first reported that Ivins was under suspicion, said the scientist had taken a massive dose of a prescription Tylenol mixed with codeine.

White House press secretary Dana Perino said President Bush was aware there were “about to be developments” in the case but did not elaborate.

“We are not at this time making any official statements or comments regarding this situation,” said Debbie Weierman, a spokeswoman for the FBI’s Washington field office, which is investigating the anthrax attacks, said Friday.

Ivins, who received three degrees including a Ph.D. from the University of Cincinnati, co-authored numerous anthrax studies, including one published in July that described efforts to treat mice deliberately exposed to anthrax. The scientists complained of the limited supply of monkeys available for testing and said testing on animals is insufficient to demonstrate how humans would respond to treatment.

Colleagues and court documents describe Ivins as a brilliant scientist who became recently began showing signs of distress. Dr. W. Russell Byrne, a who worked in the bacteriology division at Fort Detrick for 15 years, said police forcefully removed Ivins from his job recently because of fears he had become a danger to himself or others. Byrne said he said he did not believe Ivins was behind the anthrax attacks.

Maryland court documents show he recently received psychiatric treatment. Last week he was ordered to stay away from a woman he was accused of stalking and threatening to kill.

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 a colleague of Ivins’, Steven Hatfill. Hatfill’s name has for years had been associated with the attacks after investigators named him a “person of interest” in 2002.

Unusual behavior by Ivins was noted at Fort Detrick in the six months following the anthrax mailings, when 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 long stayed on Hatfill.

Henry S. Heine, a scientist who had worked with Ivins on inhalation anthrax research at Fort Detrick, said he and others on their team have testified before a federal grand jury in Washington that has been investigating the anthrax mailings for more than a year. He declined to comment on Ivins’ death.

FBI vehicles with tinted windows had watched Ivins’ home for a year, neighbor Natalie Duggan, 16, said.

“They said, ‘We’re on official business,’ ” she said.

Tom Ivins said Friday that federal officials working on the anthrax case questioned him about his brother a year and a half ago. “They said they were investigating him,” he said from Ohio, where he lives, in a CNN interview.

Ivins played keyboard and helped clean up after masses at St. John the Evangelist Roman Catholic Church in Frederick, where a dozen parishioners gathered after morning Mass to pray for him Friday.

The Rev. Richard Murphy called Ivins “a quiet man. He was always very helpful and pleasant.”

The government paid Hatfill $5.82 million to settle a lawsuit contending he was falsely accused and had been made a scapegoat for the crimes. According to one person briefed on the case, who spoke on condition of anonymity because of the ongoing investigation, the Justice Department wanted to close the Hatfill lawsuit before bringing criminal charges.

Hatfill’s lawyer, Tom Connolly, said he would not discuss the case until the FBI has time to speak with the family members of victims of the anthrax attacks.

Five people died and 17 were sickened by anthrax powder in letters that were mailed to lawmakers’ Capitol Hill offices, TV networks in New York, and tabloid newspaper offices in Florida. Two postal workers in a Washington mail facility, a New York hospital worker, a Florida photo editor and an elderly Connecticut woman were killed.

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Associated Press writers Dave Dishneau and Chrissie Thompson from Frederick, Md. and AP researchers Susan James and Jennifer Farrar in New York contributed to this story.