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

Twenty-one of the victims were brought to area hospitals following the explosion with a partial roof collapse.

The city’s fire department tweeted that nine of the victims were in critical condition, while another was in serious condition.

The trapped workers were rescued through a window. Firefighters continued to search the building Wednesday afternoon.

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.

Utility spokeswoman Stephanie Anne Weaver said work on the building’s “air handling and boiler system” likely caused the accident.

“Window washing scaffolding was comprised 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.

Mayor Brandon M. Scott released the following statement regarding the explosion at the BGE Building: 

This morning, an explosion at the BGE Building ignited a fire near the top of the building, trapping people on the surrounding scaffolding. The Baltimore City Fire Department remains on scene as the cause of the explosion is still under investigation.

Thanks to the swift action of our Fire Department, 23 people have been rescued, 21 of whom have been transported to area hospitals. Nine are in critical condition, one is in serious condition, and no deaths have been reported. I want to thank BCFD and our first responders for their life-saving work today. Without their bravery and quick response, the outcome could have been much worse. As the Fire Department’s investigation continues, my office will provide updates.