Austin police are investigating a fourth, more sophisticated explosion. A bomb, triggered by a trip wire, detonated in a southwest Austin neighborhood Sunday night.
The explosion seriously injured two young men. They're expected to be okay.
Houston's police chief is sending two bomb techs and a canine team to help Austin investigators.
The three bombs before this latest one were package bombs. Investigators believe this is the work of a serial bomber.
In the wake of the bombings, the NAACP is urging caution to members and all communities. Dr. James Douglas, president of the Houston chapter, is on the Factor to talk about it.