A private helicopter crashed in a mountain range outside Phoenix on Friday, killing all four people aboard, authorities said.

The crash occurred around 11 a.m. local time Friday and involved an MD 369FF helicopter, according to the Federal Aviation Administration.  

The Pinal County Sheriff’s Office said an eyewitness who called 911 said the helicopter struck part of a recreational slackline that had been strung up across the mountain range before falling to the bottom of the canyon.

Slacklining is an activity, similar to tightrope walking, that involves balancing on a flat piece of webbing between two anchor points, according to Slackline U.S.

The identities of the four people on board the helicopter have not been made public, but the sheriff’s office said the pilot was a 59-year-old man from Queen Creek and passengers were three female family members in their early 20s.

The helicopter had departed from Pegasus Airpark in Queen Creek, Arizona, located west of Superior, according to the sheriff’s office.

Superior is nearly 70 miles east of Phoenix.

Crews worked for several hours to access the remote crash site, located in the mountains near Telegraph Canyon, south of Superior, Arizona, according to the Pinal County Sheriff’s Office.

The National Transportation Safety Board and the FAA are investigating the crash.

A temporary flight restriction was issued by the FAA over the area “to provide a safe environment for search and rescue ops,” according to the notice.