WASHINGTON  After four years pursuing one former Army scientist on a costly false trail, F.B.I. agents investigating the deadly anthrax letters of 2001 finally zeroed in last year on a different suspect: another Army scientist from the same biodefense research center at Fort Detrick in Frederick, Md.

Over the last 18 months, even as the government battled a lawsuit filed by the first scientist, Steven J. Hatfill, investigators built a case against the second one, Bruce E. Ivins, a highly respected microbiologist who had worked for many years to design a better anthrax vaccine.

Last weekend, after learning that federal prosecutors were preparing to indict him on murder charges, Dr. Ivins, a 62-year-old father of two, took an overdose of Tylenol with codeine. He died in a Frederick hospital on Tuesday, leaving behind a grieving family and uncertainty about whether the anthrax mystery had finally been solved.

The apparent suicide of Dr. Ivins, a Red Cross volunteer and amateur juggler who had won the Defense Department’s highest civilian award in 2003, was a dramatic turn in one of the largest criminal investigations in the nation’s history. The attack, the only major act of bioterrorism on American soil, came in the jittery aftermath of the Sept. 11 attacks. It killed five people, sickened 17 others and set off a wave of panic.