* Formally conclude U.S. Army scientist was responsible

WASHINGTON,  (Reuters) – Over 1,000 possible suspects  faced scrutiny before investigators finally concluded a U.S.  Army scientist alone committed the deadly 2001 anthrax attacks,  according to Justice Department documents released yesterday.

Officially closing its investigation, the department said  various steps taken in the past year only confirmed its earlier  conclusion that the scientist, Dr. Bruce Ivins, who committed  suicide in 2008, had mailed the anthrax-laced letters.

The letters killed five people, sickened 17 others, jolted  a nation reeling from the Sept. 11 hijacked-plane attacks and  resulted in one of the FBI’s largest investigations ever.

In seven years after the attack, an “Amerithrax task force”  of investigators spent more than 600,000 work hours, conducted  some 10,000 witness interviews on six continents, and recovered  about 6,000 items of potential evidence.

The documents for the first time detailed the scope of the  work by the FBI and other investigators in scrutinizing more  than 1,000 individuals, located both in the United States and  overseas, as possible suspects.

They included physicians, scientists, researchers, a  disgruntled foreign scientist, a microbiologist who committed  suicide after the attacks and a microbiology student with  alleged ties to al Qaeda’s anthrax program.

But all of those suspects, as well an another U.S. Army  scientist, Steven Hatfill, who had been an early focus of the  investigation, were eventually ruled out and the attention  shifted to Ivins.

By 2007, investigators conclusively determined that a  single-spore batch of anthrax created and maintained by Ivins  at his laboratory in Maryland was the parent material for the  spores in the letters.

“The evidence gathered in this seven-year investigation  establishes that Dr. Bruce Ivins was the anthrax mailer,”  according to the documents, citing direct evidence about the  anthrax spores and what it called “compelling circumstantial  evidence.”

Ivins committed suicide on July 29, 2008, just as  prosecutors prepared to charge him with murder for committing  the attacks. His attorney has maintained he was innocent.

Some of the evidence involved his suspicious behavior.

“Dr. Ivins was alone in his lab for long stretches of time  in the evenings and on the weekends leading up to the anthrax  mailing events. This picture is in stark contrast to his  behavior before and after the mailings,” the department said.

It said the suicide “was the result of his final downward  slide” into depression and other mental health problems.

“Dr. Ivins profound mental health struggles provide both a  context for his motives to commit the crime and an explanation  for how he could commit such a horrific and tragic offense,”  the department said in a 92-page summary.

In the months after his suicide, investigators continued to  review thousands of e-mails going back 10 years and examined  additional evidence.

Investigators also obtained court orders allowing access to  his mental health records and interviewed mental health  providers who had treated Ivins.