A private helicopter crashed Friday in a mountainous area of Arizona, killing all four people aboard, officials said.

Those killed were the 59-year-old pilot, two 21-year-old women and a 22-year-old woman, the Pinal County Sheriff’s Office said in a post on the social platform X. Their names have not been released.

The crash took place around 11 a.m. near Telegraph Canyon, about 64 miles (103 kilometers) east of Phoenix, the sheriff’s office said. It may have been caused when the helicopter hit a “recreational slackline” that was more than half a mile (more than a kilometer) long and strung across the mountains, the office said.

“An eyewitness who called 911 reported seeing the helicopter strike a portion of the line before falling to the bottom of the canyon,” the sheriff’s office said.

The Federal Aviation Administration and the National Transportation Safety Board are investigating.

The helicopter had taken off from an airport in the town of Queen Creek, about 29 miles (47 kilometers) west of the canyon. It took crews several hours to travel by foot to the remote area where the helicopter crashed.

Flights were temporarily restricted over the area due to safety reasons, according to the sheriff’s office.

“Our prayers are with the victims and their families,” the sheriff’s office said.