Several other workers who had been caught in the debris and injured had been removed, Washington Fire and EMS said on Twitter. There were no immediate details on their condition.
Local television stations reported that the trapped worker was injured but conscious and in contact with rescuers.
Images showed dozens of firefighters swarming over piles of wood left behind by the collapse, about 5 miles (8 km) north of the Capitol building, just east of Rock Creek Park.
Last week, a condominium tower collapsed in Surfside, Florida as most residents slept. Searchers there have recovered 18 bodies and say another 145 people remain missing.
President Joe Biden traveled to the scene of the 12-story Champlain Towers South in Florida on Thursday.