Tens of thousands of letters mailed around the US could have picked up trace amounts of anthrax while passing through a contaminated postal centre, the nation’s top anthrax investigator has said.

US investigators have discovered a trace of anthrax in a letter sent to a civilian's home.

Preliminary tests have found no evidence of anthrax on the personal mail, mailbox or post office of an elderly Connecticut woman who mysteriously died of the bacterium.

A letter to US Senator Patrick Leahy was laced with billions of anthrax spores, authorities said, as a suspected case of the most deadly form of the disease mysteriously appeared in Connecticut.