Linda Weaver, Tres Caffall's mom, released a statement Monday evening:"Our family was shocked and devastated by the tragedy this afternoon. Our thoughts and prayers go to the families of the deceased and the wounded victims. We mourn them and the loss of Tres. He had been ill. It breaks our hearts his illness led to this. Please respect our privacy in our grief."As a general rule, flu doesn't lead to this sort of thing. Almost certainly mental illness.
UPDATE: A bit more discussion that confirms that he was known to be mentally ill well before this incident:
"He was crazy as hell," Richard Weaver, gunman Thomas Caffall's stepfather, told Houston station KPRC television.
"At one point, we were afraid that he was going to come up here and do something to his mother and me," Weaver said....The Eagle newspaper that he “fits the profile of a dude who might snap.”Shawn Kemp, a local acquaintance of Caffall’s, told
Caffall seemed depressed and often talked about guns and war, Kemp told the newspaper.
"I don’t know the guy well, but I’ve been around him enough to know, well, that I’m not surprised at all,” Kemp said, adding that he had heard that Caffall planned to pawn some of his guns to pay his rent.