Every day, United Parcel Service Inc. flies cargo around the world, and hardly anyone takes great notice. That was not the case Friday, in an unfolding terrorism probe that reached from Yemen to Britain and Dubai - and on to Philadelphia.

During a daylong drama, officials isolated two UPS cargo planes at Philadelphia International Airport and one at Newark Liberty International Airport.

After security sweeps, the planes and their cargo were cleared.

Hours earlier, concern over suspicious packages began in Europe, where early Friday two UPS planes departed Cologne, Germany, and Paris, headed to Philadelphia International.

"The aircraft in question were, indeed, bound for PHL," UPS spokesman Mike Mangeot said in an e-mail.

UPS Flight 218 landed in Philadelphia at 9:13 a.m. UPS Flight 201 landed around the same time, although federal officials would not provide the exact time or say which plane came from Germany and which from France.

The Federal Aviation Administration ordered a 10-minute ground stop at Philadelphia from 9:51 to 10:01 a.m. Inbound flights were briefly held up, FAA spokeswoman Arlene Salac said.

UPS is the world's largest transportation company, and its Philadelphia hub has been a gateway for countless tons of merchandise since it was built in 2005.

Philadelphia is one of four smaller UPS air-freight hubs, along with Ontario, Calif.; Rockford, Ill.; and Hartford, Conn.

United Parcel's 49.7-acre complex at the Philadelphia airport handles 80,000 packages and documents per hour headed to and from 18 states, as far west as California.

The Philadelphia complex includes a 681,000-square-foot sorting facility and a 66,000-square-foot freight facility.

UPS averages 44 daily flights in and out of Philadelphia. Last year, planes landed 1.7 million tons of cargo here, according to the company.

That represents about 57 percent of all freight at Philadelphia International, airport spokeswoman Victoria Lupica said. UPS flights usually arrive between 10 p.m. and midnight and depart between 3:30 and 4:30 a.m., Lupica said.

A small number of UPS flights - about five - arrive at other times, such as the flights Friday from Germany and France, she said.

Some of the UPS planes that land in Philadelphia crossed the ocean carrying imports; others come from U.S. cities. Sometimes, the freighters unload goods in Philadelphia that are then sent off in trucks; other times, the cargo lands in Philadelphia only to be redirected to another UPS hub, said David G. Ross, a transportation analyst who follows UPS for investors with Stifel, Nicolaus & Co. in Baltimore.

"These networks are so complex," Ross said.

UPS does not disclose to investors detailed information such as what products flow through its hubs or from where, Ross said.

Philadelphia is a natural site for a UPS hub - as Newark, N.J., is for FedEx - because it is so close to major East Coast transportation corridors, Ross said.

Deliveries of cargo from one U.S. city to another have not been a source of much growth for the industry in recent years, Ross said. The demand for imports has fueled the need for more international cargo flights, he said.

"We're getting more stuff from China now than we did 20 years ago, not as much stuff moving from Boston to Atlanta in the air," Ross said.

In a July issue listing the world's top 50 airports by cargo volume, Air Cargo World ranked Philadelphia in 40th place, behind Chicago, Los Angeles, Miami, Dallas, New York, and first-place Memphis, headquarters of FedEx, but ahead of San Francisco and Houston.

Louisville, Ky., home to UPS's central air-freight hub, was ranked sixth.

Air-security measures for cargo-only flights have been much less stringent than those enacted for cargo on passenger flights after the Sept. 11 attacks.

A law went into effect in August mandating screenings of all cargo transported on passenger planes, as part of federal legislation passed in 2007, said Stephen M. Lord, director of the homeland security and justice team at the Government Accountability Office in Washington.