According to sources, the devices were constructed by gutting a toner cartridge and installing a complete improvised explosive inside it -- a detonator, main charge and cell phone initiator. One source gave an initial estimate that 10 to 14 ounces of homemade high explosive were contained in the devices.
Officials believe that the powder may be the explosive PETN, used in the failed plots of the so-called shoe bomber and underwear bomber.
White House spokesman Robert Gibbs said that as a result of the discovery of the packages, "additional measures were taken regarding . . . flights at Newark Liberty and Philadelphia International Airports."
The discovery of the packages led to an international search of cargo planes carrying packages to the United States from Yemen today.
Authorities in Newark and Philadelphia searched UPS and FedEx cargo jets. US fighter jets also escorted Emirates Air flight 201 from Dubai into John F. Kennedy airport in New York City this afternoon. According to a statement from NORAD, the plane was determined to be "an aircraft of interest."
Gibbs said President Obama was notified of a potential terrorist threat Thursday night by John Brennan, deputy national security advisor for counterterrorism, at 10:35 p.m. "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 president vowed to enhance screening of cargo planes in the U.S., and said he had been in touch with the president of Yemen.
Brennan issued a statement thanking Saudi Arabia "for their assistance in developing information that helkped underscore the imminence of the threat emanating from Yemen." Brennan also thanked the U.K. and the United Arab Emirates for their assistance.
There had been conflicting reports about whether either of the two intercepted packages tested positive for explosives. British and US officials initially reported that tests for explosives were negative, but other American law enforcement officials said at least one, possibly both, contained explosives. In a statement late Friday, British officials said only the "suspect package" was "currently being examined."
The package being examined in the UK also included a portion of a cellphone.
British Home Secretary Theresa May said that "a suspect package was discovered during a search of a cargo flight at East Midlands airport." May said the package originated in Yemen and was addressed to a destination in the US.
A FedEx official confirmed that a suspicious package shipped from Yemen had been "confiscated" at the FedEx facility in Dubai. Local authorities confiscated the package in cooperation with the FBI.
In a statement, the Department of Homeland Security said it was taking steps to enhance air security. "Some of these security measures will be visible while others will not," said the statement.