CROCKETT, Calif. (AP) _ A helicopter hit power lines over a bridge, crashed into a waterway and sank Saturday afternoon. Five people were missing in the swirling waters, the Coast Guard said.

Authorities had located an underwater object believed to be the downed helicopter, but divers discovered it was not the wreckage, Contra Costa County Sheriff's Lt. Ron Rodrigues said.

Officials on the helicopter had been investigating a 3-day-old oil spill in Carquinez Strait, where the Sacramento River meets San Pablo Bay north of San Francisco Bay, Coast Guard spokesman Stephen Edge said.

A fisherman in a boat on the strait said he had waved to the pilot of the low-flying chopper just before it snagged power lines over the Carquinez bridge.

''As soon as (the pilot) waved, he hit this cable and he pulled his arm back in and that was it,'' said Ron Moniz, 41, of Benicia. ''The thing just dropped. It was intact. The rotors were bent up, but there was no explosion.''

''The aircraft just dropped and sank within five seconds,'' Moniz said.

The Coast Guard identified one of its officials, Lt. Carl Johnson, as among the missing. Also aboard were two people from the state Department of Fish and Game, a contract employee of the local organization Clean Bay and the pilot, Edge said.

The helicopter was a Bell 207 Jet Ranger, said Stephen Ricks, Clean Bay's manager. The organization had chartered the chopper through Helicopter Adventures in Concord, which in turn got the aircraft from another company, he said.

Crockett is 25 miles northeast of San Francisco.