I agree that DW could have carried it out alone, except that there are a couple things that make me think it may not be the case in this situation. One is that we aren't allowed to sleuth certain people -- like those who were in the convoy when he was caught -- and so we don't know that no one else was arrested. We just know that no arrests were announced. Second, LE seems convinced -- at least they were at the time the SWs were issued -- that there was more than one person involved. Perhaps it's just because they have the same false belief that a lot of people do that a number of people can't be restrained by just one person, but maybe it's because they know more than we do and there is evidence that points in that direction. For example, we learned today about a third person's DNA on the vest found in the burned car (the first two being DW and SS).
Also, there's this: "The suspect in the murders of four people inside a Washington, D.C., mansion could not have acted alone, according to a detective who was questioned for hours Monday during a court hearing in the case."
http://www.cbsnews.com/news/detective-2-d-c-mansion-murder-victims-were-strangled/