SAN FRANCISCO >> Howard Kuljian and his family were out for a walk on a damp, overcast morning at Big Lagoon beach, playing fetch with their dog Fran as 10-foot surf churned the water just feet away like a washing machine.

Signs near the beach warned of “sneaker waves,” the kind that suddenly roar ashore.

Kuljian tossed a stick that took the dog down to the water’s edge, and in an instant, authorities said, a wave swallowed it, setting off a nightmarish scramble.

“Everything kind of snowballed from there,” Coast Guard Lt. Bernie Garrigan said.

Kuljian’s 16-year-old son, Gregory, ran to save the dog, only to be captured by the surging surf himself. Kuljian, 54, followed, and then his wife, Mary Scott, 57. On shore, their 18-year-old daughter, Olivia, and Gregory’s girlfriend could only watch.

Both parents’ bodies were later recovered, but the boy — presumed dead — is still missing.

The dog eventually made it back to shore.

News of Saturday’s tragedy shocked many in the small college town of Arcata on the rough Northern California coastline about 280 miles north of San Francisco.

Students at Gregory’s high school wore green in his memory Monday.

By late afternoon, more than 1,300 people “liked” a Facebook page set up by the teenager’s friends called “Wear Green for Geddie” — using his nickname. Dozens tweeted tributes with the hash-tag (hash)WearGreenForGeddie.

“I will always remember him no matter how long,” wrote Emmalaya Owen on the Facebook page. “Especially how he was such an upbeat happy person or how he tried to put up ‘Be Happy’ propaganda posters he drew around school.”

Others were trying to come to terms with the deaths. His sister graduated last year.

“He was just a friendly guy, and everyone who knew him liked him, and his family was very close,” said Day Robins, a high school senior. She said Gregory and his family were active in school athletics and sailing.

At Big Lagoon beach, a short drive from Arcata, signs posted near the parking lot warned beachgoers not to turn their back to the surf and to pay special attention to sneaker waves.

“Because the beach is designed that way, when that 10-foot wall breaks, it surges up on the beach and surges back really fast,” said Garrigan, the Coast Guard officer. “It’s like a cyclical washing machine.”

As the family walked along the beach, Howard Kuljian threw the stick and the dog gave chase, said Dana Jones, a state parks district superintendent.

Seeing his son in the water, Kuljian leapt to action, and disappeared into the frigid water.

Gregory managed to pull himself back onto the sand, but after realizing his father was drowning, both he and his mother went in to save him.

As Olivia and the girlfriend watched in horror, a nearby bystander called police. By the time help arrived, it was too late. Jones said the officer wasn’t able to get to the family members because of the high surf.

Garrigan said the search for the teenager was stopped because a person without a wetsuit could not survive for long in the cold surf.

The Coast Guard deployed a helicopter and two motorized life boats to find the teenager, but thick coastal fog made the search difficult. The parks department also called off its search.

“When there is shorebreak like that, you don’t even have to go into the water to be pulled into the sea,” Jones said. “It’s a reminder to be real careful around the ocean.”