WASHINGTON (CNN) -- Anthrax suspect Bruce Ivins took several hours of administrative leave on the day it is believed two anthrax-laced letters were mailed, a government source said.

The source said records show Ivins reported to work at his Fort Detrick lab on the morning of September 17, 2001, but stayed only a short time before taking leave hours.

Though they have no direct proof, investigators believe Ivins used the time to drive to Princeton, New Jersey, where the letters were mailed. Princeton is about 160 miles from Frederick, Maryland, the home of Fort Detrick.

Ivins returned to the lab for a late afternoon appointment, according to the source, who asked not to be named.

An attorney for Ivins, Tom Degonia, emphasized there is no direct evidence showing Ivins mailed the letters or drove to New Jersey that day.

The U.S. government Wednesday declared Ivins the lone culprit in the anthrax attacks, saying he had a history of mental illness and that he created and mailed the spores used to kill five people.

Authorities say Ivins, a 62-year-old Army biodefense researcher at the U.S. Army Medical Research Institute for Infectious Diseases in Frederick, Maryland, committed suicide last week as they were preparing to charge him with carrying out the anthrax attacks.

In a statement Wednesday, Ivins' attorneys said the scientist was innocent and the stack of documents the government released to support its conclusion fell short of "concrete evidence."

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