WASHINGTON -- An American drone that is part of a fleet that patrols the border with Mexico has crashed off the coast of Southern California.

Customs and Border Protection says the drone was being flown on a border security mission when a mechanical problem developed about 20 miles (32 kilometres) southwest of San Diego late Monday. Spokesman Mike Friel says the crew operating the drone decided to crash it in the ocean.

The $12 million surveillance drone was one of 10 that the Department of Homeland Security uses to patrol the border. It was just one of two Predator B drones equipped with radar specifically designed to be used over the ocean.

Friel says the cause of the mechanical failure is unknown and that the remainder of the drone fleet has been temporarily grounded while the investigation into the incident continues.