Three letters contaminated with anthrax all were dated September 11, the same day as the terrorist attacks on New York and Washington, and contained anti-American and anti-Israel messages, US officials said.

More than 2,000 postal workers were tonight awaiting the results of anthrax tests after a colleague in Washington DC was diagnosed with the most serious form of the infection.

FBI detectives believe they have found the anthrax-laced letter which infected a worker at the New York Post newspaper, it was revealed today.

Investigators are trying to track the US anthrax outbreak back to a New Jersey postal carrier who may have handled the envelopes.