LOS ANGELES -- Four people were killed after a private helicopter crashed in a mountainous area of the US state of Arizona on Friday, authorities said.

The Pinal County Sheriff's Office said in a Facebook post that it had received reports of a helicopter crash near Superior, a town near the state's capital, Phoenix.

The crash took place around 11 am (1800 GMT), the sheriff's office said, noting that the "preliminary evidence indicates that a recreational slackline more than one km long had been strung across the mountain range."

"An eyewitness who called 911 reported seeing the helicopter strike a portion of the line before falling to the bottom of the canyon," it said.

Search and rescue crews reached the wreckage of the helicopter on foot and confirmed that all four people aboard were killed in the crash.

The victims included a 59-year-old man who was piloting the helicopter and three female family members, two aged 21 and one aged 22.

An investigation by the US Federal Aviation Administration and the National Transportation Safety Board is ongoing, the sheriff's office said.