SAN FRANCISCO - Authorities now believe a rogue wave, churned up by piling sea swells off the San Francisco coast, crashed ashore and swept away a 13-year-old boy Friday.
The search continued Saturday, but turned up empty. The missing boy's name has not been released.
Witnesses say he was standing with his seven friends along the shoreline at Baker Beach when the wave suddenly towered above their heads and blindsided them.
''They were taking a photograph of their group of eight and a rogue wave came and knocked them down,'' said San Francisco Fire Department spokesman Pete Howes. ''The beaches are very steep. They go from beach-side to deep water quickly.''
Other members of the group went into the water and scoured the waves for the boy but never found him. They were treated for mild hypothermia and exhaustion and released from a local hospital.
The waters off Baker Beach and nearby Ocean Beach are known for their unpredictable conditions and strong riptides that can easily take inexperienced swimmers out to sea. Just last month a fisherman drowned at Baker Beach while wading into the surf and stepping off a shallow sand shelf into deeper water.
Warning signs reading ''Hazardous surf. Swim at your own risk'' are posted at Baker Beach. Rich Weideman, a spokesman for the Golden Gate National Recreation Area which oversees the beach, confirmed that 12 people have drowned at Baker and Ocean beaches since 1990.
This time of year the conditions are often ripe for trouble.
''We had a fairly high tide. It was some of the highest tide I've seen,'' Weideman said of Friday's conditions. ''The warnings we had were between 10 and 15-foot-waves and it was going on for about 24 hours starting about 3 p.m. Friday ironically.''
That was the time when the rogue wave struck the group of teens.
The region's deadly seas claimed another life on Nov. 30 when Scott Smith, bass guitar player for the 1980s pop band Loverboy, was swept from the deck of his sailboat by a 25-foot wave four miles off the coast of Ocean Beach.
That wave was so strong it snapped off the boat's steering wheel. Smith's body was never recovered.
A high surf advisory continued through Saturday for the region, but personal responsibility is a key component for beachgoers, Weideman said.
''It's up to the person and their abilities,'' he said. ''We just want people to be cautious. It's always good to be watching the weather forecast.''
Howes added that visitors to San Francisco should watch more than the weather report - they should keep an eye peeled on the surf which can quickly change from mild to wild.
''If they're out their children or pets they should be very cautious,'' Howes said.
Despite the dangers, swells from San Francisco to Half Moon Bay about 30 miles south are coveted by big wave surfers this time of year. Hot spots nicknamed ''Mavericks'' and the ''Potato Patch,'' with their thrusting shallow ocean shelves, offer consistent 40-foot waves during the winter for those brave enough to surf them.