Cruz quite literally knew the drill when it came to active shooters and understood that the lockdown procedure would go into effect the moment gunshots were heard. He certainly did not want to find himself wandering suddenly deserted hallways, trying to force his way through locked doors.
So Cruz seems to have hatched a two-part scheme to thwart the well-rehearsed lockdown plan.
He began by waiting until just five minutes before the 2:35 p.m. dismissal time.
He then pulled a fire alarm.
The timing likely made it more difficult for the school to order and maintain a complete lockdown.