The near-miss shows that the suspected al Qaeda bomb was sophisticated enough to escape notice. It also shows how close terrorists came to getting the explosives airborne and bound for the U.S.
Intelligence officials were tipped off to a pair of explosive packages on planes in England and the United Arab Emirates early Friday morning.
After a six-hour sweep of cargo at the East Midlands airport in central England, Leicestershire police came up empty and removed the security perimeter they had set up, British aviation safety consultant Chris Yates said.
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But when officials in Dubai said they had discovered a bomb disguised as a computer printer cartridge, authorities urged the British to look again, a U.S. official said, speaking on condition of anonymity because he was not authorized to discuss the matter.
"As a direct consequence, they put the cordon back up again and looked again and found the explosives," said Yates, relying on a report given to him by an eyewitness to the searches.
President Barack Obama's counterterrorism adviser, John Brennan, called it "a very sophisticated device, in terms of how it was constructed, how it was concealed."
"It was a viable device. It was self-contained, so it could have been detonated and activated," Brennan told NBC television's "Meet the Press," adding that officials are trying to determine whether the planes or the synagogues were the intended targets.
On Friday evening, the Leicestershire police handed control of the investigation to the Metropolitan Police, the London-based agency also known as Scotland Yard.
Leicestershire police declined to answer questions Sunday about the searches, referring callers to Scotland Yard, which traditionally takes the lead in major terrorism cases in Britain.
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By Associated Press writers Matt Apuzzo and Gregory Katz