The FBI believes an anthrax-laced letter to US Senator Patrick Leahy, belatedly found last week, was written by the same person who sent a tainted letter to Senator Tom Daschle last month.

Health workers in the US are being vaccinated against smallpox, amid fears the virus may be used in a terrorist attack.

Thirty-one employees of the US Senate tested positive for exposure to a highly concentrated form of anthrax, officials said, prompting congressional leaders to order an unprecedented shutdown of some of their offices.