Explosives were found in Dubai in a US-bound parcel that bore al-Qaeda hallmarks, Dubai police said.
The parcel contained pentaerythritol trinitrate (PETN), the same explosive used in December 2009 in a failed plot to bomb a plane to Detroit.
Two cargo planes bound for the US were intercepted by security officials in the UK (photo, from bbcimg.co.uk) and Dubai on Friday.
The planes were coming from Yemen.
Intelligence agencies believe the packages are linked to Al-Qaeda in the Arabian Peninsula, based in Yemen.
The two suspect packages were described as “a credible terrorist threat” by US president Barack Obama.
According to UK prime minister David Cameron, the device in a package sent from Yemen and found on a US-bound cargo plane was designed to go off on the aircraft.
But Cameron said investigators could not yet be certain about when the device, intercepted at East Midlands Airport, was supposed to explode, although he told reporters it was believed the explosive device intercepted at East Midlands Airport was “designed to go off on the aeroplane”.
“We cannot be sure about when that was supposed to take place,” he added.
“There is no early evidence that it was meant to take place over British soil, but of course we cannot rule it out”
In Yemen, police have arrested a woman suspected of posting the packages.
She was detained in Sanaa, the country’s capital, as part of a search for a number of suspects believed to have been behind the plot, officials said. She was reportedly traced through a telephone number she had left with a cargo company.
President Ali Abdullah Saleh said the US and the United Arab Emirates had provided Yemen with information that helped identify the woman and pledged that his country would continue fighting al-Qaeda “in co-operation with its partners”.
“But we do not want anyone to interfere in Yemeni affairs by hunting down al-Qaeda,” he added.
Both packages were found on UPS and FedEx cargo planes, triggering alerts in the US, UK and Middle East. Other planes thought to contain items from Yemen were checked at US airports
The explosive devices, which triggered security alerts in the US, UK and Middle East, were apparently both inserted in printer cartridges and placed in packages addressed to synagogues in the Chicago area.
Dubai police said the explosives they found were also inside the toner cartridge of a printer, placed in a cardboard box containing English-language books and souvenirs.
The cartridge contained PETN and plastic explosives mixed with lead azide, they said. Lead azide is an explosive commonly used in detonators.
“The device was prepared in a professional manner and equipped with an electrical circuit linked to a mobile telephone [Sim] card concealed in the printer,” the police said.
US Homeland Security Secretary Janet Napolitano said she suspected al-Qaeda involvement.
“I think we would agree with that, that it does contain all the hallmarks of al-Qaeda and in particular [AQAP],” she told ABC News.
Heavily armed security patrols are on the streets of the Yemeni capital Sanaa.
Late on Friday, during a White House press conference, president Obama said : “Although we are still pursuing all the facts, we do know that the packages originated in Yemen.
“We also know that al-Qaeda in the Arabian Peninsula – a terrorist group based in Yemen – continues to plan attacks against our homeland, our citizens and our friends and allies.”
He stressed that new aviation security measures were being taken in light of the alert by the Department of Homeland Security, “including additional screening”.
Saudi Arabia provided information that helped identify the threat, the White House later said.
The UK’s Daily Telegraph reported that an MI6 officer responsible for Yemen had received a tip-off.
All shipments from Yemen have been suspended by FedEx and UPS, who said they would fully co-operate with investigators.