As they walked through the wooded area crisscrossed with skiers’ trails, the trio soon realized they weren’t the only people lost on the mountain. First they ran into a couple, then a larger group, then others who had also wandered off the marked trails at the 1,500-acre resort. The group included a half-dozen members of a high school ski club and one of the resort’s ski instructors, who was with two students, both about 5 years old.

By that night, after a five-hour, multi-agency rescue effort conducted during sub-freezing temperatures, Campanella, 22, along with 19 other resort guests and the ski instructor, were found and brought to safety. More than a dozen team members hiked, snowshoed, and skied uphill for about 5 miles in the frigid weather to reach the lost skiers and snowboarders, police said. No one was injured.

Days later the question remains: How did so many skiers get lost in the woods on a snowy evening?

Backcountry skiing — leaving marked trails for pristine slopes in the wilderness — is growing in popularity in the Northeast as winters shorten and climate change makes snow quality less predictable.

But Campanella said none of the skiers or snowboarders he spoke with left the property on purpose.

“They make it seem like we intentionally ducked some ropes and ignored some signs, and they say that we came down from the Snowshed peak, but that’s not at all what happened with us, at least,” he said. “I can’t speak for everyone, but for us, I never saw signs, never saw a rope or anything.”

The Killington resort is known as “The Beast of the East” because of its size and diverse terrain, which includes steep mogul runs, according to the skiing website On the Snow. It has seven mountain areas, including Killington Peak, which is the second-highest point in Vermont, at 4,241 feet, and has the biggest vertical drop in New England, at 3,050 feet, according to the resort.

An unlimited winter access pass at Killington can go for as $1,699, while a year round Beast 365 pass can cost more than $1,900.

It was about 2:30 p.m. when Killington police launched the recovery effort.

Authorities soon grew more alarmed as they realized how large the number of lost skiers had grown. As the sun dipped below the horizon and the darkness set in that early evening, the temperatures dropped even lower, according to the National Weather Service office in Burlington, Vt.

Law enforcement officials and rescue leaders declined to discuss the search effort.

“A special thanks should be given to all the volunteers who responded and worked this call,” the police department said in a Facebook post after the effort.

Killington police chief Whit Montgomery told the Rutland Herald the search was fairly manageable, but the weather required a quick response.

“One of the ways it becomes so dangerous is you get in there on fresh snow, it can be deep, it can be up to your waist or higher, with ski boots on or snow boots on … you start to sweat, you get wet, the temperatures drop more, hypothermia can set it; it gets pretty dangerous pretty quickly,” he said.

Kristel Killary, a spokesperson for Killington Ski Resort, said “several groups of skiers and riders,” had gone under a rope and left the resort’s perimeter on Saturday, “in violation of Killington’s policy.”

“Two of the skiers were minors and under the care of a ski instructor and that instructor was immediately terminated,” Killary said in an email.

The resort is working with Killington Search and Rescue to identify the guests who were among those lost on that Saturday so that it can revoke their ski passes, she said.

Before they got lost, Campanella said he and his friends were making the most of their day on the slopes, jumping in and out of woods and marked trails. They’d snowboarded down from Killington Peak on the Great Northern Trail and were somewhere near the junction with Killington Trail when they realized they were lost.

They soon learned they weren’t alone.

It was about 2 p.m., Campanella said in an interview Wednesday. Within to five to 10 minutes, they encountered another couple who were lost. Then four more people came hiking up the trail.

“About 20 minutes after that, we ran into another nine people,” he said.

Campanella said he thought he and his friends could have gotten out of the woods on their own, but there was concern about getting the two young children and the teenagers to safety quickly. That’s when members of the group called 911 for help.

Campanella said he was surprised to see that the ski instructor, who told Campanella she had worked for the resort for six years, was also lost.

“I was a little confused, but it kind of made me feel like it must not be that hard to end up on the wrong side and accidentally go down somewhere you’re not supposed to, because she didn’t intend to, and I don’t think she was purposely ducking ropes and not listening to signs,” Campanella said.

Dave Coppock, a member of Rescue Inc., a rural emergency medical service based in Brattleboro, Vt., happened to be in Killington cross-country skiing with his wife, Clare Coppock, when the call for help went out.

He received an emergency alert from Rescue Inc. on his cellphone while he was about 1 1/2 to 2 miles from the lost group, he said. He and four other members of the service responded, he said.

Aware that Saturday’s ideal conditions on the mountain could lead to lost skiers, Coppock was carrying a roughly 25-pound “ready pack” containing headlamps, a GPS device, maps, a compass, food and water, extra layers of clothing, a first aid kit, and other emergency supplies, he said.

Coppock, 68, who has been skiing Killington since the 1980s, made his way to the spot at the bottom of a ravine near the Buckland trailhead rescuers believed the lost people would emerge, and within a few moments of his arrival, the 18 lost skiers and snowboarders began to walk out of the woods one by one, he said.

“They were skiing in more or less the same area and kind of converged on one spot, which is a gully,” Coppock said. “It’s a brook that forms . . . almost a steep canyon, and the snow in there gets very deep.”

Coppock and his wife, Clare, helped everyone across the creek, then offered them water and food before the hike out.

Coppock said it is “unprecedented to rescue that many people at one time,” but skiers leave the designated area at Killington “all the time,” leading to many rescues, and many others who find their way out with help from the ski patrol or on their own.

“It looks like part of the ski area; it looks like they’re just going off the trail,” he said. “They see tracks . . . they think that, ‘OK, I’ll ski into the woods, but I know that if there’s tracks back there, it must mean that it’s going to lead me to the bottom of another chair lift.’”

Coppock said its nearly impossible for the resort to keep all ski areas clearly marked because wind, snow, and wildlife will knock lines over or cover them up.

“I absolutely don’t blame the resort on this,” he said.

As they descended the trail, the Coppocks and the lost skiers were joined by members of the Killington Search and Rescue team, he said. By 7:30 p.m., everyone was accounted for.

Molly Mahar, president of Ski Vermont, said the search and rescue fell during National Skiing Safety Awareness Month and highlighted the danger of leaving marked recreational areas.

“This situation provides an opportunity to remind skiers and riders to stay inbounds unless they have planned for a safe backcountry experience, which includes proper training, clothing, equipment, knowledge, and weather assessment,” Mahar said in a statement.

“Backcountry skiing is very different than skiing inbounds, and a minor mishap can quickly turn serious or even fatal,” the statement continued. “Skiers should not venture into terrain that is unknown to them without a knowledgeable guide and proper planning. If skiers require rescue, they put their rescuers’ safety at risk, too.”

Others said they too have gotten lost while skiing at Killington.

One day before the massive rescue effort, Darcy Morris, a 30-year-old musician and mother of three who lives in central Virginia, said she became disoriented while skiing Killington for her birthday.

She had been skiing the North Ridge with her husband, brother-in-law, and her husband’s two best friends when they realized the tracks they’d followed had led them astray. she said.

Morris and her group called the resort, and with guidance over the phone from its ski patrol and police, they were able to walk out of the woods on their own, she said.

“We didn’t cross under any boundary that we could recognize,” Morris said. “You know, I’ll celebrate my 30th trying to just have some fun — I’m not looking for extreme rebellion or adventure or anything like that. So to accidentally do that was like, ‘Oh, man, what did I do?’”

Jeremy C. Fox can be reached at jeremy.fox@globe.com. Follow him @jeremycfox.