"We are confident Dr. Ivins was the only person responsible" for the anthrax scare that killed five people and force thousands to receive vaccines, Jeffry Taylor, U.S. attorney for the District of Columbia, said in a news conference announcing the unsealing of documents related to the investigation.

Ivins, who committed suicide last week as the government may have been preparing to file charges against him, became a suspect in 2007, Taylor said.

Based on the information collected, "we believe we could have proven his guilt to a jury beyond a reasonable doubt."

Among the evidence was a flask of anthrax that was solely under Ivins's control and the anthrax in the flask was developed by Ivins. Also, envelopes with flawed tool markings were sold during the time frame at a Fredricksburg, Md., post office where Ivins had a box.

Ivins also had knowledge and expertise in handling a sophisticated piece of equipment that could dry pathogens and anthrax. He worked odd hours in the days preceding the attacks, often mailed packages under assumed names and spoke with a "consciousness of guilt" and had a history of mental health problems, Taylor said.

While the evidence is circumstantial, "it's compelling evidence," he said.