Exclusive: U.K. Terror Plot -- Why the Bombs Failed

-- The London bomb plot allegedly planned by a cell of doctors failed early last Friday morning because a medical syringe used as part of the firing mechanism caused a malfunction, ABC News has learned.

According to nonclassified documents reviewed by ABC News, and confirmed by multiple sources, both mobile telephones initiated firing mechanisms rigged inside a Mercedes E 300 parked several yards from the front door of Tiger Tiger nightclub failed despite multiple calls to the cell phones designed to remotely trigger the devices.

Had the fuel-air bombs successfully ignited into a superhot fireball filled with roofing nails, casualties were almost a certainty among the 500 or so patrons who partied late at the 1,700-person occupancy nightclub that perhaps best symbolizes London's vital nightlife scene.

Instead, at about 1:42 a.m., a vigilant ambulance crew on an unrelated call spotted a plume of cold propane from a slightly opened window of the car that contained patio fuel cylinders in the foot wells behind the driver and passenger seats, ABC sources said. When a bomb technician in a 90-pound Kevlar suit walked down to the vehicle to examine it, he also found a firing system rigged inside the car and another inside its trunk along with four jugs of gasoline. The technician successfully disarmed the devices..

A second Mercedes rigged with a similar incendiary device was parked several hundred yards away. Several experts on improvised explosives tactics suggested that the second device might have either been meant for patrons who escaped the first or to target rescue workers.

Within 14 hours after the plot failed, the same two men believed to have planted the bombs in London attempted what appears to have been a suicide incendiary attack on the doors to a terminal at Scotland's Glasgow Airport. That attack failed too. The vehicle failed to reach the doors, and its contents failed to ignite even after one of the occupants tried to douse the car in gasoline, setting himself on fire in the process.