"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 Sabastian Hizey of Puyallup.
The boy's father, Jessie Hizey, issued a statement to KIRO-TV 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Feld Motorsports, promised more inspections of trucks in the show and that the truck involved in the accident would be withdrawn.
The company based in Aurora, Ill., did not respond to requests for comment Saturday from The Associated Press.
A statement it released Friday night said: "Our company follows strict safety guidelines designed to protect both spectators and competitors alike."
