Hundreds of Pentagon staff was injured when the gunmen spilled a carton of Sarin nerve gas.

One of the 'gunmen' is arrested

Twenty-six employees died from exposure to the gas as the gunmen began issuing their demands.

The nightmare scenario was played out at the Pentagon - one of the world's most secure buildings - to test its contingency plans for such an attack.

In the mock attack a carton of milk stood in for the deadly Sarin gas, which was used in the 1995 gas attack in the Tokyo underground.

About 600 people took part in the 10-hour exercise codenamed Operation Cloudy Office including civilian and military police, emergency and medical personnel and the FBI.

A Pentagon staffer is "decontaminated"

John Jester, chief of defence protective services, co-ordinated the exercise. He said: "We believe we are prepared.

"We've had a response plan for quite some time. We know how to react to a chemical event, a nuclear event or a biological event."

Emergency vehicles raced to the massive military complex, in Arlington, Virginia, as hundreds of "victims" streamed out of the building to be hosed down by rapid response teams.

The mock terrorists sent out two notes to military officials. One mentioned the name of an Iraqi town that had been subjected to a gas attack, and the other referred to the Bible scripture that "you reap what you sow".

Mr Jester said: "With what's going on in the world, you have to look at all types of threats. You can't put your head in the sand about a chemical threat."

The American Defence Secretary William Cohen, whose staff were taken hostage in the exercise, has warned about the threat of chemical and biological strikes by terrorist groups.

Steps have also begun this year to organise bio-chemical emergency response teams in major American cities.

In April, a secret scenario played out by 40 federal officials concluded that the US government had neither the reserves nor organisation to deal with a simulated crisis where terrorists spread a virus along the border with Mexico.