The people who strung the line - which is used in a sport akin to tightrope walking - had filed an aviation safety notice, raising questions about the air alert system.

A helicopter appears to have crashed into a slackline strung across part of a remote Arizona canyon for an extreme tightrope-style activity, killing all four passengers.

The slackline was “more than one kilometer long,” according to the Pinal County Sheriff’s Office in Arizona, which added that a witness reported “seeing the helicopter strike a portion of the line before falling to the bottom of the canyon”.

The slackliners had filed an official aviation safety notice, known as a Notam, leading to questions about whether the pilot had seen the alert.

The cause has yet to be conclusively determined and investigators are currently probing the incident. An official report is due in 30 days.