The Austin serial bombings were difficult to stop as the perpetrator used many different triggers for the bombs such as being picked up or using a tripwire, a wire or cord is attached to some device for detecting or reacting to physical movement.
Two of the first three bombs were triggered upon being picked up, and another triggered upon being opened. The fourth bomb was activated by tripwire. Typically, a wire or cord is attached to some device for detecting or reacting to physical movement. 
