By Jeremy Hillman
BBC News, New York
Officials in New York City have begun conducting a large-scale anti-terror test aimed at simulating the effects of a major biochemical attack.
New York boosted security following attacks in London
Scientists released a harmless gas in the city's underground railway system.
Hundreds of devices across New York are now tracking the way the gas is being carried by the airflows between Manhattan's skyscrapers.
Officials hope the exercise will help in planning how and where people should be evacuated in case of a real attack.
The exercise comes as the city has raised security following the bombings London's transport system last month.
Monday is the first of six days of testing this month when gas will be released at different locations and times of day.
A spokeswoman for a programme involved in the project said "it went very well".
Around 100 scientists are working on the simulation.
The gas will be collected and analysed by hundreds of devices placed in baskets and on rooftops across the city, including in Times Square.
This exercise is one of the most complex gas trials ever conducted.