Federal officials laid out their case Wednesday against the scientist they believe orchestrated the 2001 anthrax attacks, but they also admitted that much of their evidence was circumstantial.

Bruce Ivins, a 62-year-old scientist who worked in the Army’s biodefense research laboratories at Fort Detrick, Md., committed suicide last week after learning that the Justice Department was planning to indict him for the anthrax mailings that killed five people in 2001.

On Wednesday, after days of widespread media coverage, the Justice Department and the FBI released search warrants that outlined their case against Ivins.

Jeffrey Taylor, U.S. attorney for the District of Columbia, confirmed at a press conference Wednesday that Ivins was the department’s primary — and only— suspect.

“We are convinced that Dr. Ivins was the only person responsible,” he said, adding that officials hope to wrap up the investigation and close the case soon. “We regret that we will not have the opportunity to present evidence to a jury.”

Taylor and Joe Persichini, assistant director in charge of the FBI’s Washington field office, described the investigation that led to a single flask of anthrax — which Ivins created and supervised. The substance in the mailing and the anthrax in the flask had the same DNA makeup and was unique, they said.

“That means that the spores used in the attacks was taken from that flask,” Taylor said. And anyone who wanted to access the flask, he added, had to go through Ivins.

Other evidence outlined in the search warrants include the fact that Ivins worked uncharacteristically late on the nights surrounding the attacks; was mentally unstable; and had a mailbox at the post office where investigators believe the anthrax-laced letters were sent.

In the search warrants, investigators documented his unstable mental state in a series of e-mails from several accounts, where he describes episodes of paranoia.

In a response to questions about whether investigators had any hard evidence, Taylor pointed to the flask of anthrax, calling it “effectively the murder weapon.”

The 2001 attacks hit close to home, shutting down Congressional office buildings for weeks and exposing dozens of people on the Hill to the substance. Letters were sent to both Sen. Patrick Leahy (D-Vt.) and then-Senate Majority Leader Tom Daschle (D-S.D.).