SAN FRANCISCO, Dec 19 (Reuters) - Three Japanese men died when their small plane crashed into a tank full of raw sewage in California, an employee at the flight school that operated the aircraft said on Tuesday.

Shoki Haraguchi was a flight instructor and Yoshiyuki Kato and Yasushi Miyata were students at Nice Air flight school in San Jose, California, Vicki Gonzales, the school's office manager, told Reuters in a telephone interview.

The three died on Monday when their twin-engine Beechcraft Travel Air crashed into a sewage tank at a wastewater treatment plant in the farm town of Gilroy, according to the Federal Aviation Administration, which is investigating the crash with the National Transportation Safety Board.

Workers draining the tank, about 10 to 15 feet (3 to 4.7 metres) deep, saw the bodies of the three men in the wreckage of the submerged airplane, FAA spokesman Ian Gregor said.