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The 12-year-old Redmond-area boy who was killed Wednesday while ziplining in his backyard was strangled by his helmet strap, the King County Sheriff’s Office said today.

King County sheriff’s detectives said a safety line attached to the zipline cable caught on the back of Jackson  Roos’s helmet, pulling it up and causing the helmet strap to tighten around his neck and cut off his air supply.

Detectives said the incident was a tragic accident and could not have been predicted. The case is considered closed.

Roos’ mother thought he was riding his bike around their home when she went to look for him. She found him hanging from the zip line, said King County sheriff’s spokeswoman Cindi West.

Medics attempted to revive Roos but he was pronounced dead at the scene, West said.

West said the zipline at the home in unincorporated King County is about 20 to 25 feet from the ground and about 100 feet long. It was professionally installed.