SOLANA BEACH, Calif. — It was a perfect spring morning for an ocean swim.

With the sun shining in a clear blue sky, Dave Martin and his triathlon training group swam past the surfers at Tide Beach on their regular Friday course through cool glassy waters about 150 yards out.

Somewhere below, a shark — now presumed to be a great white — was lurking, possibly on the hunt for a seal or sea lion. It struck around 7 a.m., charging at Martin from below and lifting him vertically out of the water, both legs in its jaws, its serrated teeth slicing deep, fatal gashes.

"They saw him come up out of the water, scream 'shark,' flail his arms and go back under," said Rob Hill, a member of the Triathlon Club of San Diego, who was running along the beach when the attack happened.

Two swimmers who had been 20 yards ahead raced back to help Martin, 66. They dragged him to shore in a little cove shielded by 50-foot bluffs. A lifeguard truck took Martin up to a lifeguard station on the bluff, where he was pronounced dead at 7:49 a.m.

Martin, a retired veterinarian, was the victim of a terrifying but rare attack. Only one person worldwide died in a confirmed, unprovoked shark attack last year, though the annual average in recent years is about four, according to the University of Florida's International Shark Attack File.

Authorities immediately closed eight miles of popular beaches for 72 hours, heading into a warm weekend. Emergency helicopters flew over the swells trying to track the shark, though experts said the chances of finding it were slim.

"The shark is still in the area. We're sure of that," said Joe Kellejian, mayor of Solana Beach, a quiet suburb of 13,000 people and million-dollar homes.

Martin's family members visited the lifeguard station in small groups, emerging in tears, before his body was transported to the medical examiner's office. Martin's home is just a few blocks from the beach.

A shark expert who examined Martin's body said sharks mistake humans for seals or sea lions. They attack with a single disabling charge and then retreat while their target bleeds to death.

"It's just very bad luck for that one man," said Richard Rosenblatt, a professor emeritus of marine biology at the Scripps Institution of Oceanography in San Diego.

Surfers reported seeing a stranded seal pup on the beach Friday before the attack. There could also have been groups of seals or sea lions in the kelp beds nearby.

Rosenblatt said the bite pattern on Martin's legs indicated the shark was almost certainly a great white that may have been 12 feet to 17 feet long. Female great white sharks sometimes come to Southern California waters to pup, he said.