TACOMA, Wash. — Investigators were examining pieces of the drivetrain from an oversized truck to try to learn why a Frisbee-sized chunk of metal tore loose and killed a 6-year-old boy at a Monster Jam show in the Tacoma Dome.

Bill Easterling, senior operations director for Feld Motor Sports of Aurora, Ill., told The Associated Press on Sunday that the type of accident that left little Sebastian Hizey dead was unprecedented in his 25 years in the truck show business, including 15 years with Monster Jam.

Police investigating the accident Friday night gathered loose parts of the drivetrain and the drivetrain loop, a special monster truck device that is supposed to hold the drivetrain on the vehicle, Easterling said. He said he could give no further details or description of the loose parts, including where they were found.

“I’ve never seen the loop or the drive shaft parts come off like this,” he added.

Steven Payne, a Feld spokesman, said the company is eager to obtain the parts for metallurgical testing to try to learn the cause of the accident.

Police will have no further information on the parts or other aspects of their probe before today at the earliest, Officer Mark Fulghum said.

Monster truck shows have been a staple at the Tacoma Dome for more than 20 years, featuring vehicles weighing 9,000 pounds or more and equipped with 5-foot tires. Drivers pop wheelies and engage in competition.

Feld has “a stellar safety record” with no prior significant accidents in Tacoma, said Mike Combs, city director of public assembly facilities.

On Friday night, sitting 20 to 25 feet above the arena floor, relatives and nearby spectators in a crowd of about 10,000 were splattered with blood when, as described by Jessie Hizey, of Puyallup, to The News Tribune of Tacoma, a 7- to 12-pound metal ring sheared off a 3-inch-long, 3-inch-wide chunk of his son’s skull.

Jessie Hizey said a red and silver truck, Natural High, had returned to the show after it failed to start and was hauled off the arena floor by forklift. As it was doing doughnuts, roaring around in tight circles in the mud, the metal ring flew out from under the vehicle, he said.

“You go out for a night of fun, and you lose your son,” Hizey said in a whisper, his face twisted in grief.

A man also was injured by flying debris during the show. Authorities have given no details on the man.

Easterly told the Seattle Post-Intelligencer the driver of Natural High was “absolutely devastated.”