According to Michael Isikoff at Newsweek, "The White House is staging a high-level exercise Saturday (today) to test responses to the prospect of a massive domestic terrorist attack involving IEDs (improvised explosive devices)—the same deadly roadside bombs that have been used by insurgents against the U.S. military in Iraq...."

"As part of the exercise, the officials will be handed a thick binder which lays out a scenario involving simultaneous terror attacks by “sleeper cells” of 20 to 25 individuals each dispersed in five cities across the country: New York, Washington, Chicago, Houston and Los Angeles. The officials will then be tested on how they direct their respective agencies to respond. “We’ve designed this to overtax the system, to push the system beyond the breaking point,” said one senior administration official familiar with planning for the event, who declined to be identified talking about it before it takes place...."

"The senior administration official played down that concern, saying the scenario is “not based on any existing or current intelligence” that terrorists are plotting IED attacks inside the U.S. The official noted, however, that both the World Trade Center bombing of 1993 and the Oklahoma City bombing of 1995 both involved versions of homemade bombs inside the country...."

"The Saturday “tabletop” exercise is the fourth conducted by the White House since December 2005 and which were authorized under a Homeland Security emergency planning directive. The first three involved scenarios involving a pandemic influenza, a smallpox outbreak and a massive hurricane. The excercises “absolutely helped us” to improve government responses to such disasters, the senior official said."