Actors taking part went through a gruelling process of "treatment"
The first day of simulated terror attacks in two United States cities has been hailed as a successful exercise by local officials.
The five-day test, which began on Monday with an explosion and fire in the north-western city of Seattle, was designed to test US emergency response systems.
It is the first large-scale counter-terrorism exercise since the 11 September 2001 attacks on New York and Washington.
Seattle Mayor Greg Nickels said police and firefighters had done "an outstanding job," the Chicago Tribune reported.
And city, county and federal officials in Seattle also proclaimed the drill a success.
The simulation, dubbed "Topoff 2," has a 200-page script containing some surprises not announced to the public in advance. It will cost an estimated $16m and involve 8,500 people in the United States and Canada.
If you think of it in real-life terms, it's overwhelming... it took a long time to get rescued, to get triaged, to go through decontamination
Devised by the Department of Homeland Security, the exercise began with the Seattle explosion, to which emergency crews responded within minutes.
The people of both Seattle and Chicago had plenty of warning of all this activity, with posters and news bulletins announcing the mock attacks for the last week so as to avoid panic.
Police in Seattle reported few concerned phone calls and only two injuries.
Actors playing victims were rushed to hospital where they undertook a gruelling process of decontamination before receiving medical treatment for their "wounds".
"It was real-life scary," actor Vivian Chamberlain told the Seattle Times newspaper.
"There was a car burning less than 50 feet from me, there was glass busting out, tyres burning, lots of smoke and flames."
"If you think of it in real-life terms, it's overwhelming... it took a long time to get rescued, to get triaged, to go through decontamination."
Later on Tuesday the full scale of the "attack" is to become apparent as "patients" begin arriving at Chicago area hospitals with flu-like symptoms.
Ridge says the exercise is a response to plausible threats
US Secretary of Homeland Security Tom Ridge is based in the White House during the exercise. It includes look-alikes playing President George W Bush, Vice-President Dick Cheney, and White House spokesman Ari Fleischer.
The writers of the scenario have even invented an imaginary terrorist organisation - called Glodo - to blame for the attacks.
Mr Ridge said that the simulated attacks were in response to plausible threats directed towards the US.
"Topoff 2 provides the opportunity to test our preparedness, and at the same time identify ways to improve response in the future," he said.
The exercise is planned to finish on a positive note, with suspects being arrested at the end of the week.