PHOENIX—A passenger aboard a Southwest Airlines flight found a full ammunition clip from a law enforcement officer's service weapon during a flight Tuesday from Burbank, Calif., to Phoenix, an airline spokesman said.

However, the owner of the clip remained a mystery.

TSA Administrator John Pistole said he was confident the clip didn't belong to a federal air marshal. He wouldn't speculate about who might have brought the clip onto the aircraft.

Passengers weren't in danger because nobody else on the plane had a gun that could use the bullets, Pistole added.

The passenger who found the ammunition handed it to the flight crew after finding it in flight, Southwest Airlines spokesman Paul Flaningan said.

The plane landed safely in Phoenix, and passengers on Flight 1297 were taken off and rescreened. The plane was also searched and no weapons were found.

The passenger who found the clip was met and questioned by TSA and Phoenix police at Sky Harbor International Airport.

The clip apparently was dropped accidentally by a licensed law enforcement officer who declared he had the clip, Flaningan said. The law enforcement officer was on a previous flight.

It's not clear how long the ammunition clip was on the plane or what caliber bullets it held.

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