Witnesses have described the harrowing moments after a 10-year-old boy was decapitated while riding on the world’s biggest waterslide in Kansas while other riders have told of their safety straps also coming loose .

They saw a trail of blood follow Caleb Schwab’s raft after he was hurled from it into safety netting, suffering catastrophic injuries.

His distraught little brother reportedly told Schlitterbahn WaterPark workers: “I just saw my little brother die because of one of your attractions.”

An investigation has been launched into how the incident occurred on the "Verruckt" raft ride at the Kansas City attraction.

Speaking anonymously, a source close to the inquiry confirmed the boy was decapitated.

Fellow rider Jess Sanford, from Lincoln, Nebraska, told CNN: “When I heard a noise that didn't sound like it was supposed to come from that kind of ride, that's when I turned around.

“I didn't understand what was going on so I only saw Caleb sliding down the last half of the slide, and then I saw the blood.

“They realized that he was dead, so I don't think they tried to revive him.”

Two women who are not family members were also in the raft at the time and were treated for facial injuries. The boy's parents — Republican state Rep. Scott Schwab and his wife, Michele — have not spoken publicly since the death. His funeral is scheduled for Friday.

The park reopened on Wednesday except for a large section that includes the waterslide, although its looming profile greeted visitors when they drove through the entrance.

Access to the Verruckt remained blocked by a 7-foot-high wooden fence.

Verrückt means 'insane' in German (YouTube)

Riders, who must be at least 54 inches tall, were harnessed with two nylon seatbelt-like straps — one that crossed the rider's lap, the other stretching diagonally like a car shoulder seatbelt.

Each strap is held in place by long Velcro-style straps, not by buckles. Riders are told to hold ropes inside the raft.

Despite the tragedy, visitor numbers were around average three days after, a spokesperson said on Wednesday.

Boy dies on 'world's biggest waterslide' at Kansas park

Sara Craig, who took her 14-year-old son and his friend to the park on Wednesday said: "I feel guilty having fun when a family is hurting so badly.”

She said the family rode Verruckt twice in one day a couple of weeks ago. She remembered a short video they were required to watch, though she didn't recall that it included any caveats about peril.

Ms Craig said that during her first trip down the ride with her son and one of his friends, her shoulder restraint came off, something she opted not to report to park workers.

"I didn't think much about it," she said. "You don't think you're gonna die."

So they rode it again, only to see the restraint on her son's friend also come loose by the time it was over.

She said the ride's operators sent them down the slide even though their combined weight was 393 pounds — shy of the 400 weight minimum the park advertises as a requirement. Craig described the ride as "very, very rough," so much so that "when I got off, my head hurt."

(YouTube (YouTube)

The water park passed a private inspection in June that included Verruckt, according to a document released by a Kansas state agency.

The Kansas Department of Labor provided a copy of an insurance company inspector's June 7 letter saying inspections had been completed on Wednesday.

In early tests, rafts carrying sandbags flew off the slide, prompting engineers to tear down half the ride and reconfigure some angles.

Caleb Thomas Schwab posing with his father Scott Schwab, a Kansas state lawmaker (AP)

A promotional video about building the slide includes footage of two men riding a raft down a half size test model and going slightly airborne as it crests the top of the first big hill.