The shooting is not believed to be accidental, police said at a briefing, but they did not provide any details.
After the man was found, officers from the county conducted a top-to-bottom search, police said, but did not find any suspects.
Police said a shelter-in-place request was initially made for safety reasons, but was canceled after the search.
The Skyline Towers development is an important part of the skyline of the Washington metropolitan area and can be seen for miles from the District and other jurisdictions.
The development includes two 26-story towers on the site near Route 7.