By JEFF PRICE   
Philadelphia Inquirer

Beginning about 10 o'clock tonight with a simulated radioactive explosion at or near Philadelphia International Airport, a two-day "field exercise" will test the ability of first responders in the five counties to deal with a terrorist attack on the transportation infrastructure.

Then, about 4 a.m. tomorrow, Bensalem will be hit with a similar simulated explosion, and, at an unannounced time, one of the region's waterways and a SEPTA train will also be attacked.

First responders at 20 to 25 agencies know the attacks are coming but do not know the specific targets.

"We're not being coy," U.S. Attorney Patrick Meehan said yesterday, "but the only way we benefit from this is to watch how the communication takes place to see how effective first responders are in identifying" the threats.

Brian Lynch, acting special agent in charge of the Philadelphia office of the FBI, which developed the exercise, said the public "may see certain things, but for the most part the exercise is going to be overnight, late Wednesday into early Thursday morning."

Lynch said terrorist bombings of a train in Madrid in March 2004 and of the London transit system last July identified "a vulnerability we needed to address."