In a group of over 100 hikers, she vanished and died. What happened to Diem Le Nguyen?

Diem Le Nguyen smiled and posed next to the Black Mountain peak elevation sign in San Diego County, holding up a peace sign, her hat and sunglasses shielding her from the morning sun. Twenty-four hours later, her body was discovered off a trail not far from foothill community streets in eastern San Diego County.

Now Nguyen’s family, friends and those with her on last week’s hike are left wondering how things could have gone so wrong.

“Our family is deeply devastated by the tragic and unexpected loss of our mother, Diem Le Nguyen. We are struggling to come to terms with this new reality,” Nguyen’s three sons said in a statement.

“We knew that she made it to the top with everybody else,” said Jimmy Thai, who organized the hike and streamed Nguyen’s arrival at the top June 23.

Nguyen, 50, went on a 5K hike with her sister and nephew and more than 150 people participating in a fundraiser for a local nonprofit that Sunday morning. About halfway up the mountain, her family turned back and she continued to the top with the rest of the group.

The hike was an annual pledge event to support Build a School Foundation, which supports school construction in remote areas around the world for underprivileged children, and involves 150 to 200 people hiking up Black Mountain in east San Diego County.

Thai said organizers plan the annual hike meticulously. They start early in the day to avoid extreme heat, provide a safety briefing, food and water and mark the trail with red ribbons tied on trees and white flowers. Volunteer sweepers moved behind the group to make sure no one was left behind, Thai said.

The trail, which starts at the edge of Hilltop Community Park, is a narrow, rocky footpath with surrounding brush. Several paths branch off from the main hike into nearby trails. It’s considered a moderate difficulty hike by Alltrails.com, a hike ranking website.

Signs around the trailhead warn hikers of wild animals, such as rattlesnakes, which seek shade during the day and are more active at night.

The Nighthawk trailhead in San Diego County.

(Sandra McDonald / Los Angeles Times)

On Sunday, a bouquet of flowers sat on the trailhead in memory of Nguyen.

“The day seemed typical, like any other hike that we organize. Exactly the same trail, the same mountain that we’ve been doing this thing, you know, over and over,” Thai said.

But according to the National Weather Service, the day was unusually warm. The county issued a heat advisory that was in effect from 10 a.m. June 22 until 8 p.m. June 23. The organization handed out black shirts with a heart on them, the same shirt Nguyen wore in the picture next to the elevation sign.

By 9:15 a.m., a few minutes after Nguyen was seen at the top, it had reached 98 degrees at the top of Black Mountain, more than 1,500 feet above sea level, National Weather Service meteorologist Brandt Maxwell said. Temperatures hovered in the 80s on the path up to the peak and never dipped below 67 degrees overnight.

A volunteer sweeper saw Nguyen about a quarter of a mile down the trail after she left the peak, Thai said, which would be the last known sighting of her alive.

At 10:08 a.m., Nguyen used her cellphone to call her sister and said she was tired and needed water, police said at a news conference last week. Nguyen’s nephew and another hiker climbed back up to the top in hopes they’d locate her, but were unsuccessful.

About 10:30 a.m., when the hike was scheduled to end, Nguyen’s sister told Thai her sibling hadn’t come down the mountain, he said. They passed around a photo of Nguyen to Thai’s team and collectively went back out on the mountain to search other trails in the heat.

“In the back of my mind, I’m confident that we run into her. That’s not even a doubt,” Thai told The Times in an interview. “We know this thing so well. These are well-marked trails. There are people, other hikers,” who would eventually run into Nguyen, he thought.

Flowers are left at the Nighthawk trailhead in memory of Diem Le Nguyen, who died last week after separating from her hiking group.

(Sandra McDonald / Los Angeles Times)

The day Nguyen went missing on the mountain, the skies were clear, the wind was light and humidity was high, which would have made it hard for the body to cool itself, Maxwell said. When people overheat and are unable to cool themselves effectively, they can suffer a multitude of effects, from cramps and rash to heat exhaustion and heatstroke, a potentially fatal condition that can include becoming dizzy, having difficulty breathing or losing consciousness, according to the San Diego County Health and Human Services Agency.

The group reported Nguyen missing to police at 11:30 a.m. June 23. Authorities mobilized three helicopters, a ground crew and search dogs to look for her through the day and night.

“Stay calm, we’re here with you. But rest assured, with all these resources, we will find your sister,” Thai recalled telling Nguyen’s family.

Thai livestreamed police helicopters flying above him while he and other volunteers searched nearby trails in the hours after Nguyen disappeared.

At one point, the group volunteering to help search for Nguyen grew so large that police determined they would interfere by becoming a distraction for search dogs and throw off the infrared cameras meant to spot Nguyen in the dark.

“To be honest, at that point I started getting worried because I knew that it’s been over 12 hours,” Thai said. “If somebody suffers heatstroke and lack of water, the outcome might not be good even if we find her.”

The next morning, Thai and the search team returned to the trailhead to resume the search at 5 a.m. When they returned hours later to regroup, police were waiting with bad news — they’d found a body about 9 a.m east of the trailhead, just a quarter of a mile away from the intersection of Carmel Mountain Road and Via Rimini at the base of the mountain.

The San Diego County medical examiner’s office confirmed the body was that of Nguyen on Wednesday, but had not released a cause of death as of Monday.

“Her legacy will live on with us forever, because this is the way the universe brings her to us and guides us to keep doing this. Do more hikes, build more schools and just help more children,” Thai said.

“Our mom was compassionate and selfless, and our family will forever cherish the memories that we had with her. We remember her for her resilience and we hope to carry on her legacy,” the family said in a statement.

In an interview after Nguyen’s body was discovered, Lt. Daniel Meyer, public information officer for the San Diego Police Department, told ABC 10 News that hikers should have a plan in place for the hot summer months, including not hiking alone and bringing enough water for the trail.