SOLANA BEACH, Calif. — A shark believed to be a great white killed a swimmer with a single giant bite across both legs in the ocean off San Diego County early Friday, authorities and witnesses said.

Other swimmers helped the man back to shore, but paramedics could not save him.

The victim was Dave Martin, 66, a retired veterinarian who was swimming with members of a triathlon club, said family friend Rob Hill.

The shark was almost certainly a great white and was possibly 12 feet to 17 feet long, said Richard H. Rosenblatt, professor emeritus of marine biology at the Scripps Insitution of Oceanography.

The attack occurred shortly after 7 a.m. about 150 yards offshore as the victim was swimming with nine other people, according to Solana Beach fire and lifeguard officials and the San Diego County Sheriff's Department.

Solana Beach lifeguard Craig Miller said two swimmers were about 20 yards ahead of the man when they heard him scream for help. They turned around and dragged him back to the shoreline along the narrow cove flanked by 50-foot-high bluffs topped by homes.

There was a single bite across both of the man's legs, Solana Beach Deputy Fire Chief Dismas Abelman said.

The victim was taken to a lifeguard station and given emergency treatment while an air ambulance was summoned, but bled to death before he could flown to a hospital, a sheriff's statement said.

A witness, Ira Opper of Solana Beach, described the victim as "burly and athletic." He said he was wearing a black wetsuit that was shredded on both legs. He saw one leg heavily bandaged visible bite marks on the other leg. Opper said paramedics worked on the man for at least 20 minutes.

Swimmers were ordered out of the water for miles around Solano Beach and sheriff's helicopters were sent up to scan the waters for the shark.

"The shark is still in the area. We're sure of that," Mayor Joe Kellejian said.

Hill, also a member of the Triathlon Club of San Diego, said he was running on the beach while about nine other members were in the water when the attack took place.

"They saw him come up out of the water, scream 'shark,' flail his arms and go back under," Hill said.

"The flesh was just hanging," and the man may have bled to death before he left the water, Hill said.

He said Martin lived just blocks from the beach.

"He was down here all the time," Hill said.

Hill said club members had been meeting at the beach for at least six years and never had seen a shark.

"We don't see them out here at all," he said.

However, Hill said that earlier this week he saw a seal or sea lion on the beach. Miller said a seal pup was found on the beach Friday morning before the attack and was taken to a marine animal rescue center.

The shark may have confused the wet-suited swimmers with his prey, Hill said.

Rosenblatt, the shark expert, said white sharks travel through the area, and the way the man was attacked and the "massive" but clean wounds "sounds like what a white shark would do."

White sharks hunt along the bottom, look for seal silhouettes above and then rise to attack, he told a press conference at the beach.

"A human swimmer is not too unlike a seal," he said.

Scott Bass of Encinitas, an editor at Surfer Magazine, said he was out surfing when the attack happened but didn't see it. He said helicopters started flying overhead, announcing, "There's been a fatal shark attack. Go in immediately."

Miller said eight miles of beachs would be closed to swimming for 72 hours.

Solana Beach is 14 miles northwest of San Diego.

Earlier this year, stories of shark sightings swept the coast from San Diego County north through Orange and Los Angeles counties, the Los Angeles Times reported in late March. One surfer claimed a bite mark on his surfboard was made by a great white at Bolsa Chica State Beach, but lifeguards at nearby Huntington Beach said there was no evidence of great whites in the vicinity at that time. Other shore authorities also hadn't recorded any unusual sightings.

The last fatal shark attack in California, according to data from the state Department of Fish and Game, took place on Aug. 15, 2004, in Mendocino County at Kibesillah Rock. The victim was a man diving for abalone with a friend.

On Aug. 19, 2003, a woman swimmer was killed by a great white at Avila Beach in San Luis Obispo County on the central California coast.

The last fatal shark attack along San Diego County was off Ocean Beach in April 1994.

Overall, shark attacks are extremely rare. There were 71 reported worldwide last year, up from 63 in 2006. Only one attack, in the South Pacific, was fatal, according to the University of Florida.

The university's International Shark Attack File has counted an average of 4.1 people killed by sharks annually worldwide in the last seven years.

Sharks are highly migratory, making it unlikely that Friday's attacker poses additional risk to swimmers, said George Burgess, a biologist at the university. Still, other sharks may lurk.

"It's not any more dangerous than it was yesterday or the week before," Burgess aid. "The reality is when you enter the sea it's a wilderness experience. There are animals out there that can and do occasionally do harm to us."

Sharks feed on seals and sea lions, making areas where those animals live more vulnerable to attacks, Burgess said.

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Associated Press Writers Elliot Spagat and Chelsea J. Carter in San Diego and Robert Jablon and Solvej Schou in Los Angeles contributed to this report.