The metal parts appeared to fly off the truck's drive line amid a burst of sparks as it roared past the stands where the boy was sitting.
A family member told KOMO News that the little boy was at the Monster Jam on a big family outing, and that he was having the time of his life before the accident - and that everything happened so quickly, he probably did not suffer.
But that didn't make the tragedy any easier for the family or for the people who were sitting nearby and saw the gruesome accident unfold before their eyes.
"One minute you're eating popcorn and screaming for the next guy to race. And then there's total tragedy," said Cori Barkey.
She witnessed the accident from a few rows away and is still haunted by what she saw - a little boy lying in the stands with a devastating injury, not moving, bleeding profusely, his family traumatized.
"It was just really sad. The parents had other little kids with them - they were screaming and crying. And the events staff was just standing there. It was really weird."
The truck that sprayed metal chunks into the crowd is seen moments before the accident. (Image submitted by KOMO News viewer Ashlee Foltz)
Witnesses said the red truck, dubbed the "Natural High," was driving in circles, and a spray of metal pieces came off of the truck and flew over a safety barrier. The pieces knocked the man off of his feet and hit the boy in the head, causing them to bleed profusely.
"It was just a horrendous scene," said witness Gabe Mausten. "It was a very disturbing scene."
After the boy was hit, witnesses said a man in shock rose up from his seat and began yelling, "My kid is dead! My kid is dead!" The flying debris just missed the boy's father, and he was not injured. But another man seated nearby appeared to be hit in the neck, witnesses said.
The boy and the injured man were treated at the scene and taken to an area hospital by ambulance, fire officials said. The boy later was declared dead by the Pierce County Medical Examiner's Office.
Witnesses said after the accident, people began leaving in droves but event organizers continued on with the show.
"It was kind of angering a lot of people that they didn't even stop the show," Mausten said.
One fan who was there grabbed one of the mechanical parts that came from the truck, including a circular part about a foot in diameter.
There are plenty of questions about the mishap itself, and the response to it.
Quite a few spectators, including Ashlee Foltz and her husband Joshua, wonder why the show went on, while paramedics in the stands scrambled to save lives.
"It was kinda creepy. I thought they should have at least stopped it and make everybody stay in their seats until they got them out," Foltz said. "I don't think they should have told everybody what happened. It would have been mass chaos and they wouldn't have been able to get them out."
Some people near where the boy and man were hit say that help was so slow in coming, they even started throwing bottles and cups and food onto the track to get somebody's attention.
"Accidents happen - I get that," Barkey says. "But the fact they didn't make an announcement - there could have been a doctor in the next section over."
The Tacoma police are investigating the accident, but they have not yet released their results.
The show's promoter, Illinois-based Feld Motor Sports, said they don't have a cause just yet.
"We really don't know, we have to gather the information and we are in the process of doing that now before we make that determination," said Bill Easterly of Feld Motor Sports.
The organizer also released a brief statement Friday night and then a longer one Saturday afternoon that said, in part:
"All of us at Feld Motor Sports are saddened by the accident that occurred last night at the Monster Jam show in Tacoma. ... 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.
"All of us at Feld Motor Sports and all the Monster Jam drivers and crew extend our deepest sympathies to the families involved. We will continue to investigate this incident and will provide additional information as it becomes available," the statement concluded.
The city of Tacoma released the following statement about the accident:
"The City of Tacoma and Tacoma Dome staff are deeply saddened and our hearts go out to the families of the child who died and to the man who was injured from the accident at last night's Monster Jam event.
"The Tacoma Dome takes the safety of its customers very seriously, and we are working with Feld Motor Sports, the promoters of Monster Jam, to investigate the accident and ensure the safety of guests at the remaining shows this weekend," the statement added.
There were four more Monster Jam shows scheduled at the Tacoma Dome this weekend. The shows have not been canceled, but anyone with tickets who doesn't want to attend can apply for a refund by calling Ticketmaster at 866-448-7849.
Feld Motor Sports says they have never had an accident in the stands at one of their events. But there have have been other kinds of previous accidents involving massive trucks, which can weigh more than 9,000 pounds and feature tires more than five feet high.
According to the Associated Press, truck accidents have killed five people and injured more than 40 between 1992 and 2007.
In 2007, a monster truck performing outside an Illinois auto-parts store crashed into a crowd and injured nine people.
In 2000, a Virginia woman won a $12.4 million lawsuit, after a truck accident crushed her arm. She and 12 other passengers had paid for a thrill ride in Grave Digger in 1998, when the massive vehicle flipped while doing doughnuts.
Fatalities from the 90s include: A man killed at an Illinois convention center by a souped-up Bronco; a teenager killed in 1997 by a truck that crashed through a fairgrounds fence in Idaho; and a man killed in 1994 by a monster truck that rammed through some rodeo hay bales in Oklahoma.
(Material from the Seattle Post-Intelligencer is included in this report.)