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 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 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.

“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. The building was practically empty because of the holidays and the coronavirus pandemic.