Four construction workers were injured Saturday around 11 a.m. at the State Department on C Street NW after a small “flash fire” ignited, according to District fire officials and a State Department statement.

One of the workers, whose name was not released, is in serious condition at MedStar Washington Hospital Center, said D.C. fire spokesman Lon Walls. He said the fire “appears to be accidental” but is still under investigation. Two other workers were transported to the hospital with serious but not life-threatening injuries, Walls said.

A fourth worker sustained injuries after falling off a ladder. The workers were repairing air ducts in the building, Walls said, when the fire ignited. The fire was put out before firefighters arrived, Walls said.

State department personnel were temporarily relocated but the main building was quickly reopened, according to a State Department news release.