The discovery of the packages led to an international search of cargo planes carrying packages to the United States from Yemen Friday.
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 Friday 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."
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.
"The public may recognize specific enhancements including heightened cargo screening and additional security at airports. Passengers should continue to expect an unpredictable mix of security layers that include explosives trace detection, advanced imaging technology, canine teams and pat downs, among others. As always, we remind the public to remain vigilant and report suspicious activity to local law enforcement."
A spokeswoman for the Jewish Federation of Chicago told ABC News the federation was alerted this morning and passed on the alert, advising local synagogues to take security precautions. She also said the group was not one of the targets.
The FBI's Chicago office said that the packages "in question" had been addressed to two Chicago locations, but would not provide further details. The FBI "notified the targeted institutions" by phone this morning, according to a spokesman. The White House said that the synagogue across the street from the president's home in Chicago was not one of the synagogues targeted.