LOS ANGELES — U.S. Customs and Border Protection has indefinitely grounded its remaining fleet of nine drones after operators were forced to crash a pilotless aircraft off the Southern California coast because of a mechanical problem, a spokesman said on Tuesday.

The mishap on Monday marked the second crash of one of the agency’s drones since it began using the unnamed aerial surveillance technology in 2006.

“While on patrol off the Southern California coast, the unmanned aircraft, a maritime variant of the Predator B, experienced a mechanical failure,” U.S. Customs and Border Protection spokesman Michael Friel said in a statement.

The ground-based flight crew determined the drone would be unable to return to where it originated in Sierra Vista, Arizona, and put the aircraft down in the Pacific, Friel said.

The drone hit the ocean about 20 miles southwest of San Diego at about 11:15 p.m. local time on Monday, said a Customs and Border Protection official who declined to be named. He said the decision to bring the aircraft down over water was made out of concern that it might otherwise lose power and crash on land.

The drone and systems on board were worth $12 million, the official said.

While the cause of the drone’s malfunction was not known, the agency has grounded the rest of its fleet as a precaution, and it remained unclear when flights would resume, Friel said. “We want to determine the cause of this mechanical failure (and) that will help inform the decision on what the future holds for the fleet,” he said.

The agency has nine drones left in its fleet, a U.S. Customs and Border Protection official said. All of them are Predator B models, which are made by the company General Atomics Aeronautical Systems, and they are used to spot illegal border crossers and narcotics traffickers.