Evidently, AA1012 and UA 628 landed after they couldn't get an answer from the tower and Potomac TRACON couldn't get an answer by phone. The tower only had one ATCO at that hour and the FAA is investigating in the belief he went to sleep. While landing w/o ATC radio contact wouldn't surprise me at other airports, I'm a bit surprised Potomac let them land at DCA, considering the paranoia that surrounds the Versailles on the Potomac.