Investigators open anthrax letter

The letter was opened at a sterile laboratory

Forensic experts have opened an anthrax-contaminated letter sent to an American senator in an effort to trace its origin.

The FBI released pictures showing the letter, sent to Senator Patrick Leahy, being opened in a biomedical laboratory at Fort Dentrick near Washington, with its contents being collected and wrapped in foil.

Environmental tests conducted overnight in the area outside and inside our building were negative

Fed spokesman Dave Skidmore

Investigators hope to find clues providing DNA samples, or fingerprints which may lead them to the perpetrators of anthrax attacks in the US which have killed five people since October.

In another development, the US Federal Reserve reported that the mailroom at its main building in Washington has tested negative for anthrax, after mail there initially tested positive for the deadly bacteria.

"Environmental tests conducted overnight in the area outside and inside our building were negative," Federal Reserve spokesman Dave Skidmore said.

The "Fed" - which sets US interest rates and is therefore possibly the most important financial institution in the world - had remained open, but had cancelled all public events.

Cross-contamination

Tens of thousands of letters may have picked up trace amounts of anthrax through contact with infected mail such as the letter to Senator Leahy, experts say.

The Leahy letter is the first time experts have had a chance to open a piece of mail known to be contaminated.

Protective clothing hanging on a fence outside a Senate building

The envelope contained a photocopied letter identical to that sent to Senator Tom Daschle on 9 October, with phrases such as "We have this anthrax, you die now", and "Allah is great".

The FBI says, however, that it will be weeks before their analysis will be complete.

Officials believe it is part of a batch of four similar letters all sent on the same day, and all postmarked at Trenton, New Jersey. The other recipients were NBC News and the New York Post.

The letter did not reach Mr Leahy, and was not opened earlier because of stringent safety precautions. Instead, it was placed in a sealed bag and was found in mid-November to contain anthrax.

By that time, investigators say, it had leaked enough spores to contaminate many other letters. Senator Leahy said it contained enough anthrax to kill 100,000 people.

Low risk

Dr Jeffrey Koplan, head of the Centers for Disease Control and Prevention (CDC), emphasised that the risk to any one person was extremely low.

But he admitted that cross-contamination could account for two anthrax deaths that have so far resisted explanation.

Dr Koplan's comments came as investigators said the anthrax used in mail attacks in October was virtually identical to a strain developed more than 30 years ago in a US military biological warfare programme shut down in 1969.

In another move, two Congressmen have released draft legislation calling for $3bn to be spent on measures to fight bioterrorism.