A red truck came apart while doing doughnuts during the freestyle competition of Friday night's Monster Jam show, the witnesses said. Debris from the truck flew 30 to 50 feet over a safety barrier into the stands.
"The rear axel broke and parts started flying. No one is sure if it was a part or a rock that flew up," witness Michael O'Hara told CBS Affiliate KIRO.
"Parts were falling off and a piece flew up and hit a little boy," Christine Moe told King Television of Seattle.
Police Officer Mark Fulghum said officers serving as security at the Tacoma Dome investigated the accident.
"At this point, there's nothing to indicate that there's anything criminal," Fulghum told The Associated Press on Saturday night. "Right now it looks like a tragic accident."
The Pierce County medical examiner's office identified the boy killed as Sebastin Hizey of Puyallup, Wash.
His head was just covered in blood. It was horrible. And the mom was just crying, and the dad," witness Adrian Kelley told KIRO. "They had to take the mom away … because she was freaking out."
The boy's father, Jessie Hizey, issued a statement to KIRO on Saturday that said his son was hit in the head by a Frisbee-sized piece of metal, weighing between 7 and 12 pounds.
"I cannot get the images" out of my head, the father said.
The man who was injured was taken to a hospital Friday night, but Robert McNair-Huff, community relations manager for the city, said the man's identity was not available Saturday.
Some spectators told the TV station they had to throw cups off the stands to get the attention of medics. The show continued after the two were hurt, and many spectators left.
"They just kept going," Moe said. "We grabbed our kids and just bee-lined out of there."
Laurie Deranleau, 32, a nurse from Westport, told The News Tribune, "Everybody sitting around thought they should have dropped the show and gave the family some respect. Nobody was paying attention to the show."
The Tacoma Dome was continuing with four Monster Jam shows on Saturday and Sunday.
McNair-Huff said the promoter, Feld Motor sports, promised more inspections of trucks in the show and that the truck involved in the accident would be withdrawn.
"All of us at Feld Motor Sports are saddened by the accident that occurred last night at the Monster Jam Show in Tacoma when two of our customers were seriously injured," the Aurora, Ill., company said in a statement Saturday to the AP. "Feld Motor Sports is looking into this tragic accident as the safety of all our customers is our top priority and this type of incident has never happened before in the history of Monster Jam events."