President Obama said Friday that two suspicious packages sent from Yemen and bound for the United States contained hidden explosives, discoveries that touched off a wide-scale terrorism alert including the search of two cargo planes at Philadelphia International Airport.

Obama said the packages, which were intercepted in the United Arab Emirates and England, had been addressed to "two places of Jewish worship in Chicago."

"The detail was specific and credible," said U.S. Rep. Charles Dent (R., Pa.), ranking member of the Homeland Security Committee's subcommittee on transportation security and infrastructure protection. "These were real threats."

Besides the two planes in Philadelphia, authorities searched a cargo plane at Newark Liberty International Airport and an Emirates commercial flight that two Air Force F-15 jets escorted to John F. Kennedy International Airport in New York City. All four planes had cargo originating from Yemen, authorities said, but nothing of concern was reported found.

The Associated Press reported that Friday night, officials at JFK Airport investigated a British Airways flight from London, though an airline spokeswoman said passengers disembarked as normal after authorities met the flight. Earlier in the day, police also stopped and searched a UPS truck in Brooklyn, N.Y., but found nothing dangerous.

In Philadelphia, no inbound or outbound passenger flights were affected by the searches, airport spokeswoman Victoria Lupica said.

The UPS planes were taxied to a remote area of the airport to be inspected. Police in Newark and Philadelphia also evacuated the cargo terminals at both airports "out of an abundance of caution," according to the Transportation Security Administration.

The investigation was triggered by the discovery of a suspicious powder in a toner cartridge shipped from Yemen via UPS, a federal law enforcement source told The Inquirer. The cartridge, which had wires attached, was detected during an early-morning screening at East Midlands Airport north of London. The U.A.E. package reportedly was found at a FedEx facility in Dubai.

The device discovered in England "may be some sort of" improvised explosive device, a source told The Inquirer. U.S. officials told AP late Friday that the device may have contained PETN, the explosive used in an attempted attack on a commercial airliner on Christmas.

Intelligence personnel had been monitoring a suspected plot for days, officials said. The packages in England and Dubai were discovered after a Saudi Arabian intelligence operative passed information to the United States, one official told the AP. The packages would have arrived in the United States about 4 or 5 a.m. Friday.

"The decision was not based on specific information," an official told The Inquirer, "but out of concern about potential vulnerabilities."

Federal agents still are searching for similar packages, a federal official briefed on the case said. "We're looking for packages, but it's hard to put a number on it," the official said. "It depends on how far back you go in terms of looking for packages sent from Yemen to the U.S."

Dent said he thought it was significant that the packages were sent on cargo, rather than passenger, planes. "It's probably safe to say there's been less security attention paid to cargo aircraft," he said. "It's quite clear that our enemies are probing our defenses."

Since the Sept. 11, 2001, terrorist attacks, federal officials have grappled with how to inspect air cargo without bankrupting its carriers or clogging a vital route of global trade. A 2007 Congressional Research Service report found that slightly more than a quarter of the value of goods shipped to the United States arrived by air. At the time, all passenger baggage was required to be screened, the report said, but the TSA was reluctant to say how much air cargo was inspected.

"I don't know if they're able to screen 100 percent," said Wojtek Wolfe, a national security expert and professor at Rutgers University-Camden. "It's always a balance between security and economy."

Wolfe echoed the Congressional Research report when he noted that terrorists likely would not find much value in bombing air cargo carriers such as UPS. Rather, he said, he suspected the explosives might have been meant to be assembled by someone in the U.S. into a larger device.

After the discovery of the packages, the government of Yemen "launched a full-scale investigation," said Mohammed Albasha, a Yemeni Embassy spokesman in Washington. He noted that no UPS cargo planes operate in Yemen, but sources said UPS uses subcontractors.

UPS, which operates in 200 countries and describes itself the world's largest package-delivery company, said Friday it would stop accepting packages from Yemen. The company's main hub is in Louisville, Ky., and Philadelphia is one of four U.S. regional hubs.

Yemen, the ancestral home of Osama bin Laden's father, is the base of operations for al-Qaeda on the Arabian Peninsula, a branch known for targeting Western interests in Yemen and Saudi Arabia. U.S. officials credited Saudi authorities Friday with helping them realize the "imminence of the threat emanating from Yemen." One of the group's key figures is Anwar al-Awlaki, a radical Yemeni American cleric.

John Brennan, the president's counterterrorism adviser, was not prepared to lay blame for the devices but signaled that the group was a strong suspect. "Al-Qaeda in the Arabian Peninsula has been rather open in its venom towards the United States, toward Western interests," he said. "There are a number of individuals there that we're very concerned about, so we're looking at all possibilities."