WASHINGTON—After more than eight years, federal investigators on Friday closed their files on the 2001 anthrax attacks with the formal finding that the sole person responsible for killing five people and sickening 17 others was Bruce Ivins, the government scientist who was the focus of the investigation when he committed suicide in July 2008.

The Federal Bureau of Investigation and Justice Department released more than 2,700 pages of investigative documentsand a 92-page summary of the investigation that added new details to their probe of the 2001 attacks.

According to the summary, after years of investigation and some missteps, in the summer of 2008, prosecutors in the District of Columbia U.S. attorney's office were preparing to seek a grand-jury indictment charging Mr. Ivins with using a weapon of mass destruction.

"Aware of the FBI investigation and the prospect of being indicted, Dr. Ivins took an overdose of over-the-counter medications on or about July 26, 2008, and died on July 29, 2008. Administrative and investigative steps taken in the past year toward closure of the investigation confirm the conclusion that Dr. Ivins perpetrated the anthrax letter attacks," the investigative summary reads.

Dr. Ivins was a researcher at the U.S. Army Medical Research Institute of Infectious Diseases at Fort Detrick, Md.