Police gather at the Villanova University campus where an active shooter was reported on August 21. Matt Slocum/AP

The calls seem to follow a script – a panicked and shaky caller says they are hiding in a college library , sometimes with the staccato of taped gunshots in the background.

First responders then flood the campus, searching for mass casualties and preparing to kill or be killed.

Students snap into their active shooter training when their phones light up with automated warning messages from their universities. They move to shelter silently behind locked or barricaded doors while sending texts to their loved ones, not knowing if the messages could be their last.

The minutes tick by, marked by pounding hearts and sweaty palms, tears in terror-filled eyes, until the

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