Somebody felt very wronged by this family, you wouldn't think it would be that hard to figure who was harboring the major grudge. Then again, everybody that knows may be dead. People talk though, especially young adults, so some of their closest friends or neighbors are bound to know who pulled the trigger.
The two guys being roughed up and beat up a little before they were shot says a lot. Figure out who was mad at them, and the rest of it will fall into place.
Whoever did this had been to all four homes sometime previously. They knew exactly where to go, and who they were gunning for.
This wasn't some sort of a sanctioned hit or a contract where the motive was to exterminate the entire family in order to send a message.