A bomb found on a US-bound cargo plane was powerful enough to bring down an aircraft, British authorities said on Saturday, as forces in Yemen searched for suspected al Qaeda militants behind a plot to bomb Jewish targets in Chicago.
"I can confirm the device was viable and could have exploded. The target may have been an aircraft and had it detonated the aircraft could have been brought down," British Home Secretary Theresa May said.
Two parcels sent from Yemen and containing explosives were intercepted in Dubai and Britain on Friday, triggering broad travel disruptions and a massive international investigation.
Officials said the parcel bombs had the hallmarks of al Qaeda, and in particular al Qaeda in the Arabian Peninsula, and appeared to include the same explosives used in a failed attempt to blow up a US jetliner on Christmas Day last year.
In Washington, US Homeland Security Secretary Janet Napolitano also said authorities were checking whether other packages had been sent before the two that were intercepted.
"We're doing some reverse engineering as it were to identify other packages from Yemen," she said on NBC News.
US President Barack Obama addressed the nation on Friday, saying US authorities would spare no effort to find the source of the packages which he called a "credible terrorist threat" aimed at two synagogues.
One of the packages was found on a United Parcel Service cargo plane at East Midlands Airport, about 160 miles (260 km) north of London. The other was discovered at a FedEx Corp facility in Dubai.
The parcel intercepted in Dubai contained a bomb hidden in a printer, said Dubai police, whose experts defused the device.
"The parcel was prepared in a professional way where a closed electrical circuit was connected to a mobile phone SIM card hidden inside the printer," a Dubai police statement said.
The plot originating in Yemen further heightened security concerns about the unstable Arab state, seen by the West as the home of al Qaeda's most inventive and audacious affiliate.
Yemen-based al Qaeda in the Arabian Peninsula, or AQAP, and one of its leading figures, US-born Muslim cleric Anwar al-Awlak, have been priority US targets since it took responsibility for a failed plot to blow up a US jet on Christmas Day last year with explosives a Nigerian man hid in his underwear.
Dubai police said they found pentaerythritol trinitrate in a printer and cartridge, the same chemical explosive used in that failed plot.
There was no immediate claim or responsibility for the parcels but US officials suspect AQAP which is affiliated with al Qaeda, whose militants killed 3,000 people using hijacked airliners in the Sept 11, 2001 attacks on the United States
The White House said Saudi Arabia had helped to identify the threat from Yemen while Britain and the United Arab Emirates also provided information.
Obama called Saudi Arabia's King Abdullah and British Prime Minister David Cameron on the parcel bomb plot on Saturday.