(CNN) -- A suspicious package discovered at the United Kingdom's East Midlands airport contained "viable explosives," and could have brought down an aircraft upon detonation, British Home Secretary Theresa May said Saturday.
The preliminary U.K. investigation indicates that the target may have been an aircraft, May said, but authorities do not believe the perpetrators would have known the location of the device when they detonated it.
May added that there is no indication of any other attack on British soil, and the threat level in the United Kingdom remains unchanged and at severe threat level.
Earlier Saturday, U.S. Secretary of Homeland Security Janet Napolitano told CNN Saturday that the plan to send explosives on a flight to the United States has the "hallmarks of al Qaeda,"
"We know that the perpetrators of this -- and it has the hallmarks of al Qaeda, the AQAP -- they are constantly trying things to test our system," she said, referring to al Qaeda in the Arabian Peninsula.
The suspected terror plot involved two suspicious packages found abroad, addressed to Jewish organizations in the United States, that contained considerable amounts of explosive material.
The devices were designed to be detonated by a cell phone, a source close to the investigation told CNN.
"We also know that al Qaeda in the Arabian Peninsula ... continues to plan attacks against our homeland, our citizens, and our friends and allies," he said during a press briefing on the incident Friday.
"Initial examination of those packages has determined they do apparently contain explosive material," Obama said.
The devices were "professionally" loaded and connected using an electric circuit to a mobile phone chip tucked in a printer, Dubai police told WAM, the official news agency for the United Arab Emirates.
They were packed in toner cartridges and designed to be detonated by a cell phone, a source close to the investigation told CNN.
Police said they were tipped off about the possibility of an explosive device in postal packages onboard a FedEx flight from Yemen headed to Dubai.
The Saudi government provided U.S. officials with tracking numbers of the two packages, enabling quick tracing to the United Kingdom and Dubai, a source told CNN.
"What happened is you have great information sharing from the Saudis," Napolitano said Saturday. "We were immediately able to work across the globe to get these packages segregated."
Pressed by CNN's T.J. Holmes on whether the United States would have known about the plot had it not been for the Saudis, she said, "We certainly got the heads-up from the Saudis."
"I don't want to go into other intelligence," she said. "That would be inappropriate. I don't play the 'what if' game. What if the Saudis hadn't told us? We share information. We share information like this across the globe. The ability for passengers to travel safely is a global issue and all countries of the world need to be involved here."
Mohammed Albasha, a Yemen Embassy spokesman in Washington, said no UPS or FedEx flights take off or land in Yemen. He said his government is investigating, but it was too early to speculate or reach any conclusions.
Yemen is cooperating with regional and international partners, including the United States and the United Kingdom, the spokesman said.
A Yemeni government official, who was not authorized to speak to the media, said Saturday that police would close UPS and FedEx offices in the country as part of the investigation. Packages bound to the United States from Yemen "are being inspected and scrutinized as part of the investigation," the official said.
He said workers at local UPS and FedEx offices were questioned, as were other cargo workers.
The package found at East Midlands Airport contained a "manipulated" toner cartridge and had white powder on it as well as wires and a circuit board, a law enforcement source said Friday. A similar package set to be shipped on a FedEx cargo plane was discovered in Dubai, officials there said.
The source close to the investigation said the type of material found in the devices was PETN, a highly explosive organic compound belonging to the same chemical family as nitroglycerin. Six grams of PETN are enough to blow a hole in the fuselage of an aircraft.
PETN was allegedly one of the components of the bomb concealed by Umar Farouk AbdulMutallab, the Nigerian man accused of trying to set off an explosion aboard a Northwest Airlines flight as it approached Detroit, Michigan, on December 25.
Abdul Mutallab is alleged to have been carrying 80 grams of PETN in that botched attack -- also believed to be the workings of al Qaeda in the Arabian Peninsula.
By comparison, the source said the two devices found Friday contained multiple times more PETN.
Both packages bore addresses in the United States, "specifically two places of Jewish worship in Chicago," Obama said.
The packages led to increased searches of cargo planes and trucks in several U.S. cities, said law enforcement sources with detailed knowledge of the investigation.
White House counterterrorism chief John Brennan said that "the materials that were found and the device that was discovered were intended to do harm."
The Transportation Security Administration on Friday stopped all packages originating from Yemen, and shipping companies UPS, FedEx and DHL all said they were complying with the order. May said Saturday that all cargo into or through the United Kingdom originating in Yemen was halted as well.
Counterterrorism officials are taking the threat "very seriously," Obama said. The Department of Homeland Security said it has taken measures to intensify security.
"We have put in place enhanced protections for cargo and passengers emanating from Yemen and making sure we identify all packages coming from there," Napolitano said Saturday. She stressed that officials are acting out of "an abundance of caution."