(CNN) -- A federal law enforcement officer mistakenly left a loaded gun magazine that was found Tuesday on a Southwest Airlines plane, officials said.

The discovery was made after the flight from Burbank, California, to Phoenix, Arizona, landed, a member of a CNN crew aboard the plane said.

The head of the Transportation Security Administration said the unnamed law officer will be given remedial training.

"It belonged to a member of federal law enforcement," John Pistole said of the magazine, adding it was not believed to belong to a federal air marshal.

"The full magazine was found in a back seat pocket," a TSA official told CNN. "We believe it was left by a law enforcement officer on a flight that originated in San Jose (California) and landed in Burbank. The officer was not an air marshal and we are trying to establish contact with the agent."

"The item was immediately turned over to the crew working the flight, who called in the local authorities to handle the investigation," a Southwest Airlines statement said. "The passengers who were remaining on that flight were rescreened and the plane was thoroughly inspected before returning to service."

The officer followed the proper protocols to get the gun on the plane and authorities were in the process of returning the misplaced magazine to the officer, said Southwest spokeswoman Brandy King.

"We were just taken by surprise," said CNN photographer Gregg Canes, who was headed to Phoenix along with producer Sara Weisfeldt to cover former Alaska Gov. Sarah Palin's book signing.

He said that after the plane had landed, a child behind them, who had been seated on his mother's lap, was crawling across the other seats on the row toward the window when his foot knocked something onto the floor. A flight attendant picked it up, he said, and he could see it was a gun magazine.

Canes said the flight attendant would not let them take pictures of it.

"It was actually almost funny, given the amount of scrutiny that we've been paying to the [Transportation Security Administration] and personal security. It seemed almost funny to see a magazine with bullets in it just sort of lying on the floor of a commercial jetliner," Canes said.

Passengers were allowed to deplane, he said.

CNN's Ed Lavandera, Mike Ahlers and Paul Vercammen contributed to this report.