The biggest anti-terrorism exercise held in the US is under way to test its response to attacks involving biological and chemical weapons.
Emergency services are acting as though the scenarios are real
More than 10,000 people are taking part in the five-day exercise, which also involves Canada and the UK.
A simulated biological attack by terrorists in New Jersey on Monday coincided with a mock chemical attack at a port on the coast of Connecticut.
On Tuesday, the focus of the exercise will shift to hospitals.
"The point of this is not to design a simulation that makes us 'look good' because we were able to figure out how to pre-package everything that we wanted to do," US Homeland Security Secretary Michael Chertoff said.
"The point is to actually drive at the areas where we think there are potential questions or criticisms, to really push there in order to learn more lessons."
Although no real weapons or biological agents are used, federal and local officials as well as emergency services respond as if it is the real thing.
Witnesses describe police and emergency workers moving tentatively towards a blazing car on a university campus in New Jersey.
A hose pipe is spotted hanging out of a rear window and a commercial sprayer lies nearby, presumably used to disperse the fake biological agent.
The police commander orders caution and the deployment of biological suits. People flee the area in apparent distress, reporting sudden flu-like symptoms.
In the Connecticut port of New London, dozens of people pretending to be victims lie under overturned buses in a scenario mocked up to be the result of a chemical attack.
The Department of Homeland Security says both types of attack are plausible but neither was chosen because of any specific intelligence.
Officials in both Canada and Britain are involved in the $16m exercise known as TOPOFF 3, for Top Official.
TOPOFF 2 was held in May 2003 and involved 8,500 people in the US and Canada.
It was the first large-scale counter-terrorism exercise after the 11 September 2001 attacks on New York and Washington.