The King County Sheriff's office said Friday that Jackson Roos died when a safety line caught the back of his helmet and pulled it up. It caused the helmet strap to tighten around his neck and cut off his air supply.
First responders attempted to revive Roos, but he was pronounced dead at the scene, said King County sheriff's spokeswoman Cindi West.
West said the zip line on the home's vast grounds in unincorporated King County is about 20 to 25 feet from the ground and about 100 feet long. It was professionally installed.
Detectives say the accident could not have been predicted.
Roos' mother thought he was riding his bike around their home Wednesday when she went to look for him. She found him hanging from the zip line.