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.

Preliminary evidence indicated a slackline of more than 1 kilometer in length was strung recreationally across a canyon.

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.