A mechanic working for a party bus company in San Francisco’s Bayview district was killed Thursday when a bus he was working on rolled back on top of him, state safety officials said today.

Walter Pietila, 58, was killed in an accident reported at 11:59 a.m. Thursday at a maintenance yard owned by California Experience LLC at 355 Selby St., according to Dean Fryer, spokesman for the state Division of Occupational Safety and Health.

The company operates Cali Party Bus, which offers party and charter buses in the Bay Area as well as Los Angeles and San Diego, according to its website. Company officials were not immediately available today to comment on the accident.

Fryer said Cal/OSHA’s initial investigation determined Pietila had put a bus up on a ramp to look underneath it when the vehicle somehow rolled back on top of him.

As part of its investigation, Cal/OSHA will be looking at what procedures should have been followed to make sure the bus didn’t move while it was being worked on, as well as the company’s training policies for its employees, Fryer said.

“We’ve got to dig into this a bit,” he said.