BERLIN » A United Airlines flight to San Francisco was ordered back shortly after takeoff from Frankfurt airport Friday because of a bomb threat, German authorities said.

After the jet landed back in Frankfurt, all luggage and people on board were searched but no explosives were found, a German federal police spokesman said. Nobody was detained in the incident.

"Police received a threat notice warning of a bomb attack," spokesman Christian Altenhofen told The Associated Press. "We ordered the plane back to Frankfurt, where it landed safely."

"It was a very specific threat and you have to keep in mind the date," he said, referring to the 14th anniversary of the Sept. 11 terror attacks. "It made us take the threat seriously."

Rob Tobias, a passenger aboard Flight 902, said the captain announced about 15 minutes into the flight that it was turning around because of security concerns.

"Everyone calm and Frankfurt police and fire professional," he said in a message via Twitter. On the ground, "all passengers and their luggage screened and swabbed for explosives. Also frisked. Passports also closely checked."

Altenhofen said Friday’s flight was canceled by the airline and has been rescheduled for Saturday. Police are continuing to investigate the source of the threat, which was sent by email, he said.