SAN JOSE — Climbing 2,500 feet toward the spectacular peaks of Sequoia National Park, 16 students and five leaders from San Jose State headed toward a sparkling alpine lake nestled in a glacially carved granite cirque.

On Saturday afternoon, some of the backpackers decided to swim to a rock jutting out of Eagle Lake. At some point, they realized that classmate William “Billy” Nguyen — a 19-year-old fitness buff and one of the student leaders — was no longer with them.

He didn’t surface. Frantic, the hikers sent an SOS message. How could the avid hiker and outdoors lover have sunk into the lake?

On Sunday, after a search that spanned two days, park rangers found Nguyen’s body in the lake.

The tragedy has spawned shock and disbelief.

“Everybody’s devastated,” said Jeff Cable of Saratoga, whose daughter was Nguyen’s classmate at Prospect High in Saratoga and who mentored Nguyen in photography. “He was genuinely an amazing kid. He had a passion for everything.”

Tragically, Nguyen’s family didn’t find out about his death until his mother called Sequoia National Park midday Monday, when he hadn’t returned home as expected. His mother learned over the telephone what had happened, said park spokesman Zach Behrens.

Among the various agencies involved in the case, the Santa Clara County Medical Examiner’s Office had been designated as the one responsible for informing the family.

An office employee visited the victim’s house three times Sunday. “The house had the appearance that somebody may be home,” Capt. William Oberst said, but no one answered the door.

Once the visits failed, the medical examiner’s office did not call, because it did not have a phone number for the family, he said.

In a letter to the San Jose State community, President Mary Papazian said that she spoke with Nguyen’s mother on Monday. Papazian wrote, “I am heartbroken for the Nguyen family and for all who knew and loved their son.”

The sketchy details mystify Nguyen’s friends. “He was so in shape and so aware of his body,” Cable said. Nguyen was not a risk-taker. “He was one of these kids who knew his limits.”

Nguyen, a kinesiology major, had attended a yearlong course as a freshman to learn the basics of backpacking to become a student assistant in the outdoors program, San Jose State spokeswoman Pat Lopes Harris said.

He also worked at 24-hour Fitness as a personal trainer, according to university classmate Tiffany Ha.

Nguyen had looked forward to helping lead the trip, part of San Jose State’s Outdoor Adventures program. The group, which had a wilderness permit to camp at nearby Mosquito Lakes, had set out on the 3-1/2-mile hike from the Mineral King trailhead.

After Nguyen disappeared, the group sent an SOS message at about 3 p.m. Saturday through a satellite messenger device, alerting park rangers of a possible drowning.

Three rangers hiked in from the Mineral King ranger station below. In the lake’s 52-degree waters, “they knew it was a fatality,” said Behrens.

After conducting interviews, the rangers and backpackers hiked back down to the trailhead.

On Sunday, rangers equipped with diving equipment returned and searched the lake. About 4 p.m., they found Nguyen’s body, which was taken by helicopter to park headquarters at Ash Mountain, and then to the Tulare County Coroner’s office in Visalia.

Nguyen’s backpacking companions returned to San Jose on Sunday.

“Billy was one of the most genuine, inspirational, and down to earth guy I’ve ever met,” Ha wrote in a Facebook message. “He always motivated me to keep pushing myself at the times where I wanted to give up. His loving and caring personality will never be forgotten.”

Cable, who said Nguyen’s friends are heartbroken, posted a blog about the quiet and confident young man with a once-bright future.

“Billy, I hope, wherever you are, that you can find the best trails and take the best pictures ever. I feel lucky to have met you,” Cable wrote. “You left us way too soon, even if it was doing what you loved the most.”