FRANKFURT, Germany (CBS SF) — A bomb threat prompted the evacuation of a plane heading from Frankfurt, Germany to San Francisco on Friday, a law enforcement source tells CBS.

United flight 902 returned to Frankfurt shortly after take-off at the direction of the German authorities, according to United Airlines.

The plane landed safely at 3:39 p.m. local time with 333 passengers and 18 crew members on board.

All luggage and people on board were searched but nothing was found. Nobody was detained in the incident.

Several passengers took photos as they were leaving the plane.

The flight was then canceled as the crew exceeded their legally permitted duty hours, a United spokesperson said.

Customer service representatives at Frankfurt is making arrangements for passengers to complete their journeys.