RICHMOND, Calif. >>  A federal investigator said Monday that a midair collision between two planes that ended with one crashing into the San Francisco Bay over the weekend occurred when one pilot attempted a passing maneuver.

The lead National Transportation Safety Board investigator, Howard Plagens, said the pilot of a vintage Hawker Sea Fury TMK 20 pulled up to the left side of a travelling companion flying a Cessna 210. The Sea Fury’s pilot heard a “thump” and immediately focused on trying to fly his own plane to land safely.

Plagens said the pilot saw the Cessna going down but did not see it crash.

“Obviously, he’s still shaken up,” said Plagens, who interviewed the surviving pilot twice.

The Contra Costa County Sheriff’s Office said searchers located the Cessna under 13 feet of water about 1 1/2 miles off the Richmond shoreline east of San Francisco. The crashed plane and its pilot haven’t been recovered, and officials haven’t identified the two pilots.

Plagens inspected the Hawker, which suffered tail damage, and said he’s awaiting the recovery of the Cessna to continue his investigation.

It was not immediately clear why the planes were flying so close together during the passing maneuver.

John Cox, CEO of the consulting firm Safety Operating Systems and a pilot with 44 years of experience, said the distance that pilots should maintain from nearby planes depends on whether the planes are flying together in formation.

“If they are not flying in formation, basically several hundred feet,” Cox told The Associated Press. If they are flying in formation, that distance may shrink to as little as 30 or 40 feet, he said.

The overtaking plane — in this case, the Sea Fury — is responsible for maintaining separation, but it’s possible that the surviving pilot didn’t see how close he was to the Cessna, Cox said.

The collision occurred at about 4 p.m. Sunday near the Richmond-San Rafael Bridge.

Witnesses at Point San Pablo Yacht Harbor told the San Francisco Chronicle that the Cessna spiraled out of control and crashed into the choppy water. Debris was found in the bay after the collision.

The Sea Fury’s pilot landed at Eagle’s Nest Airport in the small city of Ione in Amador County, said Federal Aviation Administration spokesman Ian Gregor. The Sea Fury’s occupants — a husband and wife — weren’t hurt. It was unclear how many people were in the Cessna.

Both planes had departed from Eagle’s Nest Airport to participate in the Pacific Coast Dream Machines, an annual festival at Half Moon Bay Airport that features a variety of planes, motorcycles and cars. Both planes left Half Moon Bay, about 20 miles south of San Francisco, and were on their return flight.

FAA records indicate the Sea Fury, a vintage British fighter plane, is registered to Sanders Aeronautics Inc. in Ione. A man who answered the phone at the company’s listed number declined to comment.

Sanders Aeronautics’ website said the family-run company specializes in aircraft restoration, and brothers Dennis and Brian Sanders are avid air racers.