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.
The first three packages were not mailed, but instead placed near victims' homes. 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. The fifth bomb was triggered in a sorting facility in Schertz, and the sixth was discovered and disarmed in a similar sorting facility in Austin. 
