Nearly two dozen people were injured Saturday night when a wooden platform collapsed during a birthday party at a San Diego gym, officials said.

The 21 children and two adults gathered at Vault PK in Barrio Logan, an indoor gym that specializes in parkour — a military obstacle course training — when the celebration turned into horror just before 8 p.m., FOX5 San Diego reported. The platform, which was 10 feet by 30 feet, collapsed while it was dangling about 10 feet above ground, striking several people who were standing below it.

“It was a freak accident,” Zachary Smith, who was standing on the platform at the time it fell, told the Los Angeles Times.

Smith added that the platform was a type of “viewing area” and that a stairwell below had buckled before the platform toppled. Children were also running up and down the stairwell to get pizza just before the incident, another parent told the newspaper. A snapchat video obtained by FOX5 shows broken wooden planks and shattered glass covering the floor while parents move children to a safer area.

At least 21 children were hospitalized, and at least four treated for major, including spinal, injuries, San Diego Fire Department’s Deputy Chief Steve Wright told reporters Saturday night. The other 17 children suffered minor injuries.

Two women, ages 72 and 46, were also hospitalized in the incident, according to FOX5. The severity of their injuries is unclear.

Wright said it’s unclear if the number of people or the construction of the platform contributed to its collapse. Officials are investigating the incident.