Looks like it's wide open at this point -- but if forced to guess, I'd go with the Aryan Brotherhood.
I remember seeing some mention in the Dallas media that Mark Hasse (the prosecutor who was murdered 2 months ago) had worked on some cases involving that gang. It was also mentioned by ABC News:
http://abcnews.go.com/Blotter/prosec...1#.UVjt5r4o6Um
Remember the thug who recently murdered the Colorado prison chief executive? He was the recent parolee who died in a shootout with sheriff's deputies about 50 miles NW of Dallas, and he was reportedly involved with the Aryan Brotherhood.
Since he was apparently on his way to the Dallas area, could he have been intending to hook up with like-minded killers and hatemongers, and could he and his cohorts have been involved with planning both of the murders?
That seems to be an angle that will be looked at very closely.