A picture released by Dubai police shows parts of a printer with explosives loaded into its toner cartridge. It was found in Dubai in a cargo plane from Yemen. (AP)

Oct. 30: A package shipped from Yemen and bound for the US on a cargo jet that was intercepted in Britain yesterday contained an explosive device powerful enough to bring down the plane, British authorities said today.

“I can confirm the device was viable and could have exploded,” British home secretary Theresa May said. “The target may have been an aircraft and had it detonated the aircraft could have been brought down.”

A day after two packages containing explosives addressed to synagogues in Chicago were discovered, one in Britain and the other in Dubai, setting off a broad terror alert, Janet Napolitano, the secretary of Homeland Security, said that the plot “has the hallmarks of Al Qaida”. Officials on three continents, meanwhile, continued to search for other potentially dangerous packages shipped from Yemen.

It was not yet clear whether the packages were intended for the synagogues or whether the intent was to cause mass destruction in the air.

“We do not believe that the perpetrators of the attack would have known the location of the device when it was planned to explode,” May said, speaking to reporters after leading a meeting of the British government’s COBRA committee, which oversees Britain’s response to terrorist threats. Yesterday, President Obama said that the explosives represented a “credible terrorist threat” to the US, and in television interviews this morning, Napolitano went a step further.

“I think we would agree with that, that it does contain all the hallmarks of al Qaida and in particular al Qaida AP,” she said, referring to al Qaida in the Arabian Peninsula.

Yemen’s security forces today appeared to be in a state of heightened alert, as investigators questioned cargo employees at the airport.

Yemeni officials have declined to comment on the plot, saying only that they are investigating. But new checkpoints appeared in the capital, Sanaa, today, with officers checking the identity cards of drivers and pedestrians.

Last week, Yemen’s interior ministry announced a ban on carrying weapons in public, amid rumours of possible car-bomb attacks by the regional Qaida branch.

At the FedEx offices in Sanaa, national security officers stood guard behind a glass wall and barred reporters from entering or asking questions.

The government denied a report circulating early today that it had found 24 additional suspect packages, but at the international airport, investigators were questioning employees of FedEx and UPS, the cargo companies that were used to send the packages that contained explosives, airport officials said.

Napolitano and the police in Dubai today confirmed that the bomb discovered in Dubai, in cargo from Yemen bound for the US, contained the explosive PETN, the same chemical explosive in the bomb sewn into the underwear of the Nigerian man who tried to blow up an airliner over Detroit last December 25.

That plot, too, was hatched in Yemen, a country that is regarded as one of the most significant fronts in the battle with extremists.

The discovery yesterday of the explosives packed in toner cartridges for computer printers was based on a tip from Saudi intelligence officials, American officials said. The British newspaper The Daily Telegraph reported in its Saturday editions that the plot was discovered “after a tipoff” to an MI6 agent “responsible for Yemen”.

The Dubai police said that tests showed the printer cartridge also contained lead azide, an explosive compound that can be used in bomb detonators.

The Dubai police said they were tipped off to the device by a call from abroad but did not name the country. The police said that the tip warned of the possibility of an explosive device hidden in postal packages onboard a FedEx flight originating from the Yemeni capital of Sanaa to Dubai, according to a statement released by the official state news agency WAM and reported by The Associated Press.