A Navy mine-hunting helicopter that blew up in flight with the apparent loss of all eight people aboard was one of the troubled Sea Stallion family of choppers that have been involved in several fatal crashes.

Search teams found seven flight helmets bobbing on the water and debris from the MH-53H Sea Dragon shortly after it exploded in a thick fog Monday morning about 12 miles from the Golden Gate Bridge, but there were no signs of survivors, Coast Guard Lt. Cmdr. Jack Hardin said.The Navy said the helicopter carried only small arms and some flares, and a military expert said some type of electrical problem may have combined with a fuel leak to produce an explosion.

Witness Frank Rescino, skipper of the fishing boat Lovely Martha, said there was a "tremendous explosion. There were a lot of pieces of helicopter floating around. It was kind of eerie."

The Coast Guard cutters Blackhaw and Romain and the Navy vessel Vanguard continued the search through the night, but the crews were not hopeful.

"We expect no survivors," Coast Guard Petty Officer Thomas Cowan said.

About 25 minutes after leaving Alameda, the downed craft's sister aircraft saw smoke in the sky. A civilian ship reported hearing two loud explosions, and its crew saw the helicopter fall out of the sky, Navy spokeswoman Virginia Felker said.

Andy Lightbody, editorial director of the magazine International Defense Images, said an explosion in flight could indicate a possible fuel leak combined with an electrical surge.