Fifteen people were missing or unaccounted for at Grand Canyon National Park in Arizona, park officials said, after a flash flood buried trails under rock slides, washed out footbridges and cut off access to a remote area on Saturday, forcing hikers to be evacuated by helicopter.

The National Park Service on Sunday was seeking members of the public who knew of hikers or backpackers who were in Bright Angel Canyon and the Phantom Ranch area on Saturday “as well as anyone who had a campground reservation in the affected corridor,” the service said in a statement.

“These were individuals either on a backcountry itinerary in the inner canyon or believed to be day hiking along the North Kaibab/Phantom Ranch area,” Joelle Baird, a spokeswoman for Grand Canyon National Park, said in an email on Sunday.

The park revised an earlier estimate of those unaccounted for or missing to 15, from 20.

Park workers and the Arizona Department of Public Safety “are continuing evacuation and response efforts and working to account for individuals who may have been in the affected area,” the Park Service said.

Two helicopters from the Park Service and the state’s Department of Public Safety searched the North Kaibab Trail corridor as well as the Colorado River from the mouth of Bright Angel Creek to the Deer Creek Narrows on Sunday morning, Ms. Baird said.

“No missing individuals were located during these flights,” she said.

One of the missing hikers is John Giusti, a father of three children and chiropractor from Granbury, Tex.

Mr. Giusti, 46, was hiking the Grand Canyon for the first time with two of his friends, his younger sister Valerie Gibbs said in an interview.

It is unclear if those friends remain missing, but in a separate Facebook post that was later deleted on his business page, his wife, Janet Giusti, said that the hiking group he was with “is among those currently reported missing and unaccounted for.”

“​We ask our ChiroHabit community to please lift up my husband, his hiking group, and our families in your thoughts and prayers during this time,” the post said.

Ms. Gibbs described her brother as a fit and active person who did a lot of hiking and running. She said that relatives were last able to get in contact with him on Saturday around 2 p.m., before the floods began.

Mr. Giusti was documenting his trip to the Grand Canyon on his business Facebook page, where he shared photos on Friday from the first day of his trip.

Ms. Gibbs said she does not know much else.

“I just know they found 100 people, and he wasn’t one of them,” she said.

At least 62 people were evacuated from the tourist spot after the flooding caused significant damage on Saturday afternoon, park officials said in a separate statement.

Those who were evacuated were transported to the Maswik Lodge food court on the South Rim, where the American Red Cross was helping. No injuries have been reported, park officials said.

“Nearly all footbridges spanning Bright Angel Creek were destroyed, eliminating pedestrian access across the creek,” the Park Service said.

At Bright Angel campground, Daniel Evans, 43, was stretched out with a book when he heard the first sounds of trouble at about 11 a.m. on Saturday. There was a rumble that sounded like a freight train, and then he noticed that Bright Angel Creek was suddenly a torrent of logs and debris.

At around 1 p.m., he said, black clouds swarmed above the canyon walls and “it just started dumping.” As the downpour continued, the rangers ordered the campground to evacuate.

Mr. Evans said he had just enough time to pack up his tent and bag and flee to a mule corral, where about 30 hikers, park staff and construction workers stood in piles of mule dung, watching the swollen river rise. Some hikers had lost their phones, backpacks, tents and even shoes in the flood.

With the trails out of the canyon washed out or buried under rockslides, Mr. Evans said, his group waited for three or four hours for rescue helicopters to arrive.

David Gregory, 59, was at the nearby Phantom Ranch, a lodge at the bottom of the canyon, when the flooding occurred. He said they could see the Colorado River, which “looked chocolate brown” and was choked with debris. The streams that fed into it, he said, were black.

Mr. Gregory heard that a bridge his party had used to cross the creek had been washed away minutes after they passed.

“If we had all been on that bridge, we would have been knocked into the creek and then washed into the Colorado River,” he said from a hotel in Flagstaff, Ariz.

He and his friends were also eventually ferried out to safety by state trooper helicopters.

Scott Bailey, who runs Stronghold Engineering, the firm whose employees were in the area updating the park’s aging water delivery system, said more than 20 workers were among those evacuated.

“Everybody’s been accounted for,” he said.

Some of the company’s temporary housing in the Bright Angel Creek area, erected while they worked on the waterline project, was washed away, and the area lost power and communications, Mr. Bailey said. He was eager to return to assess the damage and the potential delays to the project.

Officials warned of additional thunderstorms and heavy rainfall on Sunday and Monday that “could result in additional flash flooding, debris flows, rockfall and changing trail and river conditions.”

The Park Service said that Phantom Ranch, Bright Angel Campground, the Phantom Ranch Canteen and the entire North Kaibab Trail from the North Kaibab Trailhead to Phantom Ranch were closed until further notice.

This is a developing story and will be updated.

Georgia Gee and Susan C. Beachy contributed research.