Packages containing explosive materials were intercepted in the United Kingdom and Dubai on its way from Yemen to the United States, officials said Friday.
President Barack Obama says the terror probe into suspicious packages believed to be linked to a plot by Yemen's al Qaeda branch is a :credible terrorist threat."
Yemen's al Qaeda branch is the same group responsible for an attempted bombing of a U.S.-bound airliner last Christmas.
An Emirates Airlines passenger jet carrying cargo from Yemen was escorted from the Canadian border to New York City by two military fighter jets, U.S. officials said. They said it was a precautionary action.
Intelligence officials have been monitoring the suspected plot for days, an official said. The packages were discovered in England and Dubai late Thursday after a foreign intelligence service picked up information related to Yemen and passed it on to the U.S., one official said.
An official United Arab Emirates security source said the package discovered there was a device containing explosive materials. This does not mean they were actual bombs, CBS News reports. Some tests were negative but the latest tests are positive for at least some explosives.
No explosives have been found in the U.S.
The two packages were addressed to Chicago religious sites, Chicago FBI spokesman Ross Rice said. Both were sent from the same address in Yemen to Jewish organizations in Chicago, U.S. officials said. At least one was a synagogue, reports CBS News national security correspondent David Martin.
The package in England, discovered aboard a plane in East Midlands, north of London, contained a printer toner cartridge with wires and powder. It was found during routine screening of cargo in England, prompting authorities to scour three planes and a truck in the United States on Friday, U.S. officials said.
A source told CBS News that the cartridges in the U.K. and Dubai contained a syringe, powder and cell phone components. (Scroll down to see a photo of one of the cartridges.)
"The president directed U.S. intelligence and law enforcement agencies and the Department of Homeland Security to take steps to ensure the safety and security of the American people, and to determine whether these threats are a part of any additional terrorist plotting," the White House said in a statement.
A Joint Terrorism Task Force source tells CBS News that investigators are looking for between 10-20 packages shipped out of the UPS office in Sanna, Yemen during the same time frame.
Yemeni officials said they had launched a terrorism investigation, and Scotland Yard said its investigators were testing a number of items seized from the plane in East Midlands.
In the U.S., searches were conducted in Philadelphia, Newark, N.J., and New York City. Local officials said all of the suspicious items and planes that were searched had been given the "all clear."
"As a precaution, DHS has taken a number of steps to enhance security," the Homeland Security Department said in a statement. "Some of these security measures will be visible while others will not."
Since the failed Christmas bombing of a Detroit-bound airliner, Yemen has been a focus for U.S. counterterrorism officials. Before that attack, the U.S. regarded al Qaeda's branch in Yemen as primarily a threat in the region, not to the United States.
The Yemen branch known as al Qaeda in the Arabian Peninsula has since become a leading source of terrorist propaganda and recruiting. Authorities believe about 300 al Qaeda members or cells operate in Yemen.
The Yemeni government has stepped up counterterrorism operations, with help from the U.S. military and intelligence officials. Mohammed Shayba, general-director of the state airline's cargo department, said the government is conducting an investigation.
"Those in charge are in constant meetings and they are investigating and taking the issue seriously," he told The Associated Press.
CBS News Special Report (Friday Morning):