U.S. officials say they believe two packages bound for the United States contained the same powerful explosive used in the failed Christmas Day airline bombing.
The officials said full testing has not been completed but initial indications are the packages contained PETN, a chemical that was also a component of shoe bomber Richard Reid's explosive in 2001.
The packages were seized in Dubai and England on Friday.
President Barack Obama called it a credible terrorist threat.
PETN is the primary ingredient in detonating cords used for industrial explosions and is popular among terrorist groups. It is a white powder. Officials have confirmed the package in England contained a white powder.
White House Homeland Security Adviser John Brennan says the two explosive packages bound for the United States were intended to carry out an attack and could have done harm.
Brennan says the packages have been made inert and are no longer dangerous.
Brennan stopped short of linking the plot to al-Qaida's Yemen branch but said anyone who is associated with the group is a subject of concern.
UPS said it is immediately suspending shipments out of Yemen after authorities thwarted the terrorist attack against U.S. synagogues. White House Homeland Security Adviser John Brennan says the government is considering ways to step up security on shipments out of the country.
The White House says President Barack Obama's weekend travel plans to campaign ahead of the midterm elections will not be affected by a terror probe on three continents.
White House spokesman Robert Gibbs says the president still plans to visit five states in the next three days. That includes stops in Philadelphia and Chicago, both cities involved in the investigation into suspicious packages sent from to the U.S. from Yemen.
Packages containing explosives in Dubai and England were addressed to Jewish organizations in the Chicago area. A plane was also investigated at Philadelphia's airport because it was believed to contain cargo from Yemen.
Gibbs says that while the president is taking the threat seriously, he doesn't think there's any reason for the public to change their own travel plans.
The president said the American public should be confident in the government's counterterror efforts.
A U.S. official says authorities are investigating whether a string of suspicious packages was a dry run for a plot to send bombs through the mail.
A suspicious package containing a toner cartridge with wires and powder was found during routine screening of cargo in the United Kingdom, prompting authorities to scour three planes and a truck in the United States on Friday.
Searches were conducted in Philadelphia, Newark, N.J., and New York City, but no explosives were found. All the packages believed to be suspicious came from Yemen and were being sent via UPS.
Yemen is the home of the al-Qaida branch that claimed responsibility for an attempted bombing of a U.S.-bound airliner on Christmas.
The official spoke on condition of anonymity to discuss the ongoing investigation.
Officials for FedEx say the company has confiscated a suspicious package in Dubai that was shipped from Yemen and is cooperating with the FBI.
Memphis-based FedEx Corp. has embargoed all shipments from Dubai indefinitely. Company spokeswoman Sandra Munoz said Friday she could not estimate how many packages might be involved or discuss what was suspicious about the package.
She said the package originated in Yemen but referred all further questions to the FBI.
Otherwise, Munoz said, FedEx operations were running normally.
STATEMENT FROM WHITE HOUSE PRESS SECRETARY ROBERT GIBBS:
Last night, intelligence and law enforcement agencies discovered potential suspicious packages on two planes in transit to the United States. Based on close cooperation among U.S. government agencies and with our foreign allies and partners, authorities were able to identify and examine two suspicious packages, one in London and one in Dubai. Both of these packages originated from Yemen. As a result of security precautions triggered by this threat, the additional measures were taken regarding the flights at Newark Liberty and Philadelphia International Airports.