Washington - Federal prosecutors investigating the 2001 anthrax attacks were planning to indict and seek the death penalty against a top Army microbiologist in connection with anthrax mailings that killed five people. The scientist, who was developing a vaccine against the deadly toxin, 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.

    U.S. officials, who spoke on condition of anonymity because of the sensitivity of the ongoing grand jury investigation, said prosecutors were closing in on Ivins, 62. They were planning an indictment that would have sought the death penalty for the attacks, which killed five people, crippled the postal system and traumatized a nation still reeling from the Sept. 11 attacks.

    Authorities were investigating whether Ivins released the anthrax as a way to test his vaccine, officials said. The Justice Department has not yet decided whether to close the investigation, officials said, meaning it's 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 was "hounded" by aggressive FBI agents who raided his home twice, said Dr. W. Russell Byrne, a colleague who worked in the bacteriology division of the Fort Detrick research facility for 15 years. Byrne said Ivins was forcefully removed from his job by local police recently because of fears that he had become a danger to himself or others. The investigation led to Ivins being hospitalized for depression earlier this month, Byrne said.

    He said he does not believe Ivins was behind the anthrax attacks.

    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.

    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.

    Ivins was the co-author of numerous anthrax studies, including one on a treatment for inhalation anthrax published in the July 7 issue of the journal Antimicrobial Agents and Chemotherapy.

    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.

    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.

    "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.

    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 Writer Dave Dishneau from Hagerstown, Md. and AP researcher Susan James in New York contributed to this story.