BALTIMORE (AP) — An explosion in a high-rise building in Baltimore injured 23 people Wednesday and temporarily trapped a window-washing crew on dangling scaffolding.

Twenty-one of the victims — all construction workers — were brought to area hospitals following the explosion, which caused a partial roof collapse. The city’s fire department tweeted that at least nine of the victims were in critical condition, while another was in serious condition.

The trapped workers were rescued through a window.

The firefighters’ union tweeted that evidence points to an explosion on the 16th floor of the downtown building, where the offices of Baltimore Gas and Electric Company are located.

The fire department said the cause of the explosion remains under investigation. Utility spokeswoman Stephanie Anne Weaver said work on the building’s “air handling and boiler system” likely caused the accident.

“Window washing scaffolding was compromised because of the incident and a window washing crew was pulled to safety by the fire department,” Weaver said in a statement.

Weaver said no utility employees were injured. The building was largely empty because of the holidays and the coronavirus pandemic. Its gas service was not active because of the construction work.

Weaver said the building has been deemed structurally sound following an inspection and repairs are planned.