The grassy venue inside Toronto's Downsview Park was a flurry of activity late Saturday afternoon. As staff went about their final preparations, a crush of Radiohead fans waited to flood the gates for the sold-out evening concert.

Inside the beer tent, Oana Damian, 23, and a handful of other workers had just sat down for a meeting with their employer around 4 p.m. when she heard a loud, crackling sound — like “fireworks” — coming from the stage area.

Standing nearby, Irene Constantin, another beer-tent worker, said the mangled metal and scaffolding “looked like toothpicks.”

“Everything went quiet for a few seconds and then people started jumping over the tables, running towards the stage,” said Damian, recalling the dozens of workers who ran to help.

Minutes later, one stage worker was dead and three others injured. The concert, which was scheduled to begin at 7:30 p.m., was cancelled immediately.

It remained unclear Saturday night what caused the stage to collapse, but police said emergency crews were working with the provincial Ministry of Labour to determine what led to the fatal accident.

Toronto police have sent three forensic specialists to investigate the accident. A Ministry of Labour spokesperson confirmed two ministry inspectors and one engineer are assessing the scene.

The International Alliance of Theatrical Stage Employees union said Saturday it was not involved in the construction of the stage.

“Regardless of who was involved, the stage collapse is a tragic event and we offer our most sincere condolences to the family of the deceased, and our thoughts are with those who were injured,” said the union's Jim Brett.

Fire crews arrived at the scene within minutes of the collapse and began to scour the stage after hearing that a worker was trapped beneath the debris. According to Tony Bellavance, platoon chief with Toronto Fire north command, crews used inflatable bags to prop up the debris and pull the man from the wreckage.

The man, in his 30s, was pronounced dead at the scene, killed by a “heavy, crushing injury.” Police would not release the man's name until his next of kin were notified.

Another man, 45, was taken to Sunnybrook hospital with non-life-threatening injuries. Paramedics treated two other male victims for minor injuries at the scene; they were later released.

Roughly 40,000 fans were expected to attend the concert, Radiohead's first in Toronto since a 2008 appearance at the Molson Amphitheatre.

The concert was the final date of an 11-show North American tour that started in late May. The band played at Montreal's Bell Centre Friday night.

News of the cancellation left fans, some of whom had flown in from across the continent, with mixed emotions.

Drew Walley of Minneapolis arrived in Toronto on Thursday.

“Obviously I'm disappointed because it happened,” the 38-year-old said of the cancellation.

“But my own disappointment is overshadowed by the one person who lost his life.”

Almost immediately after the concert was cancelled, ticket company Live Nation promised full refunds. Concert tickets sold for $81 on Ticketmaster and were available Saturday for as much as $125.

Natalie Racz, 22, had just arrived at Downsview Park when emergency crews pulled in.

Racz has the Radiohead bear logo tattooed on her arm and was disappointed to hear that the show was cancelled.

“Someone's dead, so it's a bigger tragedy than missing a concert,” said Racz, adding that she saw at least 2,000 people quietly evacuate the park, escorted by security.

Rumours swirled in the aftermath of the tragedy that Radiohead would join the Flaming Lips at a free concert at Yonge-Dundas Square on Saturday night.

Though it didn't happen, the Flaming Lips' played Radiohead's “Knives Out” after issuing their condolences for the dead and injured.

“This unthinkable thing that happened today, we can't all help but be affected by it,” said Wayne Coyne, lead singer of the Flaming Lips. “Peace be with their hearts tonight.”

Downsview Park, a roughly 240-hectare expanse in North York, has played host to several high-profile concerts and events in recent years. Pope John Paul II said Mass at the site in 1984 and 2002. The site hosts Edgefest, a huge music festival, on July 14.

Saturday's collapse follows a series of stage accidents at Canadian concerts in the past five years.

A stage fell during a Cheap Trick concert at Ottawa's Bluesfest last year, injuring three.

In 2009, the Big Valley Jamboree main stage, in Camrose, Alta., collapsed, killing one person and injuring 15.

With files from Antonia Zerbisias and Ben Rayner

Josh Tapper can be reached at jtapper@thestar.ca

Note: This article has been edited from a previous version that mistakenly said the June 16 Radiohead concert was part of the North By Northeast Music Festival. North By Northeast did not organize the Radiohead show.