(Reuters) -An explosion shook a mostly empty downtown Baltimore office building on Wednesday, injuring 23 people, at least nine of them critically, and leaving two window washers dangling from a scaffold outside the structure, authorities said.

Of the 23 people rescued from the Baltimore Gas and Electric Co building after the explosion and partial collapse of its roof, 21 were taken to area hospitals, and two declined additional treatment, the Baltimore City Fire Department said.

Nine of the injured were in critical condition and one was in serious condition, the fire department said in a statement.

The window washers were rescued after dangling several stories high when their scaffold partially gave way, the local firefighters’ union said.

Firefighters quickly extinguished a blaze on the 16th floor where evidence suggested the explosion occurred, Local 734 of the International Association of Fire Fighters said in a statement.

The fire department said it was investigating the cause of the blast and fire, but Baltimore Gas and Electric said construction work being done on the building’s air handling and boiler system “likely caused the incident.”

“The building was largely empty due to the upcoming holidays and the pandemic,” said the company, which is owned by Exelon Corp.

(Reporting by Peter Szekely in New York; Editing by Lisa Shumaker and Howard Goller)