I've heard people guess it's a terrorist, or 2 more Columbine-like kids. One lady I know guessed it was another Tim McVeigh type.
I think it's an American grown, whacked out middle aged white guy. I know the media is denying this has racial overtones, but I think it probably does. I don't think teenaged kids would be able to travel those distances, day and night hours, rack up that many miles, without parents or teachers, or friends becoming suspicious.
As for this tall tale about the police not wanting to release a sketch prematurely, I don't buy it. Law enforcement would never hold back a sketch that might alert the public to a particular kind of face, a sketch that could at least cut by half the number of plausible suspects people could watch for. Think about it, not releasing a sketch might cause more lives to be lost, it just doesn't make sense they would hold back.
Law enforcement, probably for good reason, is spinning this yarn. Either they don't have a sketch but want the guy to think they do, or the sketch would alarm the public further, perhaps into thinking it was a racial thing.
I'm just guessing, but I admit, I'm skeptical of what the media is saying and not saying, even more skeptical that law enforcement is telling the truth at these press conferences.
That brings me to three maybe's. Either they are jihadists, mercenaries hired by jihadists or, like you say, young thrill killers who have acquired a taste for it. The third option seems to be far fetched simply because they haven't made any mistakes except for one piece of brass that I know of.