One story I have read says that he phoned WWE on Sunday, saying he would not be able to make Vengeance as his family were ill, spitting up blood.
If this is true, then it would suggest poisoning or some sort of highly contagious disease.

Another (non-wrestling) board that I visit has a thread similar to this, but they have already jumped to the conclusion it was some kinda suicide pact or something.

I'll hold judgment on that until all the facts are in!
Also, police have issued a statement that they are investigating it as homicide, but they were not shot to death!