Scientists have concluded that anthrax sent through the mail last year was less than two years old, leading investigators to believe that whoever sent the germs could make more, according to a published report.

"It's modern," a government official told The New York Times in a story to be published Sunday. "It was grown, and therefore it can be grown again and again."

The age of the anthrax that killed five people and sickened more than a dozen last fall gives credence to the theory that the person who put anthrax-laced letters in the mail is connected to a microbiology laboratory and may have used new equipment, government officials who spoke on condition of anonymity told the newspaper.

Establishing the age casts doubt on a theory that the mailer obtained an old laboratory sample of anthrax from a strain identified in 1981, the Times reported.

The FBI used radiocarbon dating to determine the anthrax was fresh. The method measures how much radioactive carbon a living thing loses since it died, or in the case of anthrax spores, since they became inactive.

The FBI has said the suspected mailer is likely a male loner, with scientific knowledge, a grudge against society and a familiarity with the Trenton, N.J., area, where the letters were postmarked. Officials remain uncertain if he is a U.S. citizen.

Officials say they have a list of about 50 suspects, the newspaper reported.