It has been reported former US president Mr Bill Clinton has been sent salmonella in the post.
The package was received at Mr Clinton's office in Harlem, New York, three weeks ago and was today being investigated.
It contained vials of a substance which initially tested positive for salmonella, the bacteria which causes food poisoning.
But Mr Clinton did not come into contact with the bug and the worker who opened the package was not affected, CNNreported.
Investigators do not believe the package is related to the anthrax letters which have caused panic across America.
CNNreported that a source close to the investigation said: "We believe it is a crank letter."
The investigators are working on the theory that the substance in the vials had turned into salmonella when it spoiled as it was in the post on its way to the former president's office.