GILROY -- Authorities continued to drain a sewage tank at a Gilroy waste facility Tuesday, uncovering the wreckage of a small plane that was believed to be carrying the bodies of three Japanese tourists.

A witness spotted the plane going down just before 4 p.m. Monday, according to a Santa Clara County Sheriff's Department dispatcher.

Emergency workers couldn't immediately get to the wreckage because it was submerged in raw sewage about 20 feet deep, authorities said. It was too dangerous for even a dive team to go in and check for bodies or identifying information on the plane, they said.

Workers were draining the pond at the South County Regional Wastewater Authority's treatment plant to get to the plane, but that was expected to continue through Tuesday.

Gilroy police Sgt. Kurt Svardal said that after several hours of work, more than 13 feet of sewage inside the tank remained to be pumped out before the airplane could be accessed safely.

Sewage in the rectangular tank -- measuring 20 feet deep and 100 feet long -- was completely obscuring the aircraft.

"You cannot see this plane," Svardal said. "And it's completely submerged."

Federal investigators were scheduled to begin work Tuesday at the site, Svardal said.

A flight log and a Japanese passport were recovered near the wreckage, said Ian Gregor, a spokesman for the Federal Aviation Administration.

A flight school in San Jose reported Monday afternoon that a twin-engine aircraft -- carrying an instructor and two students, all Japanese nationals -- was due back at Reid-Hillview Airport at 5 p.m. but never arrived. Authorities, however, could not confirm if that was the plane involved, Gregor said.

There were no calls for help made to air traffic controllers before the crash, Gregor said.

Both the FAA and the National Transportation Safety Board were expected to investigate the crash Tuesday.