They had an undercover operative deep inside the group of at least 30 terrorists who had developed a simple, effective and potentially devastating plan to close the world's busiest airport, Heathrow, by carrying on small but deadly bombs concealed in sports drink containers and detonated by the flash of a disposable camera, intelligence and law enforcement officials told ABC News.Even with an undercover operative on the inside, the British may not have known the full scope of the plot until it grew close to fruition, and authorities learned that the terrorists intended to assemble their bombs onboard at least six and as many as nine or 10 passenger jets that would depart from that airport. The goal was to deal out mass death and destruction and a crippling blow to Heathrow, a top al Qaeda target since September 11th. The airlines targeted included United, Continental and American, and the cities to which flights would have headed included New York, Washington, Los Angeles, and Miami -- all spots popular with British tourists as well as with Americans returning from trips abroad, sources said.The planes were not to be blown up as they arrived at the cities, but in mid flight.When the arrests were made Thursday morning, the plotters had reached the point of identifying airlines and routes, but had not yet picked flight numbers. An arrest in Pakistan two days ago prompted authorities to begin to their round-up of suspects. It appears the arrest in Pakistan would have quickly tipped the suspected terrorists to the undercover probe, and perhaps caused them to disperse, so authorities moved quickly. But five key members of the cell -- described by authorities as the ringleaders -- remain at large. The hunt for them, inside the U.K. and by intelligence and law enforcement authorities around the world, continues.The intent by al Qaeda-inspired terrorists to destroy Heathrow's business and cripple England's economy dates back several years. One other plot has been thwarted by British authorities in the past. The idea of using multiple planes exploding in midair to create fear dates back further to the inspiration of master bomber Ramzi Yousef, who in 1994 first tried to knock multiple jets from the sky.This latest plot, however, appears homegrown. The profile of the bombers -- young, longtime residents or citizens, inspired by al Qaeda and perhaps loosely linked to the old hierarchical terror group -- is the one that authorities have come to fear.