Photo shows a metal part that spectators say flew from the truck and apparently struck the little boy. It was found in the stands near the boy's seat.
Police gathered loose parts of the drive train and the drive train loop, a special monster truck device that is supposed to hold the drive train on the vehicle, Bill Easterling, senior operations director for Feld Motor Sports of Aurora, Ill., told The Associated Press on Sunday.
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," said Easterling, whose company is the promoter of the show.
Tacoma police Officer Mark W. Fulghum said no further information on the parts or other aspects of the investigation would be available before Monday at the earliest.
The second spectator struck by debris was taken to a hospital. Friends of the man told KOMO News that he remains in serious condition from injuries to his neck and face. His identity has not been released.
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.
The boy's father, Jesse Hizey of Puyallup, said a metal ring weighing 7 to 12 pounds sheared off part his son's skull. They and other relatives were sitting in grandstands 20 to 25 feet above the arena floor.
Jesse Hizey said the red and silver truck, Natural High, at first failed to start and was hauled away by forklift. It returned to the show, and the metal ring flew from under the truck as it was spinning in tight circles in the mud.
"You go out for a night of fun, and you lose your son," Hizey said in a whisper, his face twisted in grief.
Easterly told the Seattle Post-Intelligencer the driver of Natural High was "absolutely devastated."
The truck was not in the show Saturday and phone calls to the home of the owner, Kelvin Ramer of Watsonville, Calif., did not go through Sunday.
Monster Jams continued with four scheduled weekend shows. At the first of two Saturday shows, a moment of silence was observed and the lights were dimmed. Many would-be ticket buyers walked away in disappointment after being told that tickets for all 12,000 seats had been sold.
"They removed the truck that has the issue from the show, so we feel comfortable moving forward," Deputy City Manager Reynaldo A. Arellano said.
"I think it's just kind of a freak accident," spectator Ron Jutte said. "I feel pretty safe."