Certainly it is unusual and we will run it down, but there's nothing that links this to the anthrax letters

Secret Service spokesman

An aide opened the package, which arrived three weeks ago at Mr Clinton's office in the Harlem area of New York.

The former president was nowhere around at the time.

Mr Mackin said that the vials contained an unknown substance which had, in two of them, fermented into salmonella.

He added that "certain writings" were included in the package but would not comment further.

No-one has been taken ill as a result of handling the package.

Symptoms

Infection with the salmonella bacteria can cause fever, abdominal pain, diarrhoea, nausea or vomiting.

It can become life-threatening - especially if the infection spreads to other parts of the body such as the blood or bone.

Salmonella food poisoning reportedly affects over a million Americans every year.

The bacteria was also used in a biological attack in the state of Oregon in 1994.

Members of a religious cult succeeded in poisoning local restaurant salad bars, injuring more than 700 people. No one is believed to have died as a result of the attack.