A teen gunman killed three children and a teacher at Silver Rail Elementary Thursday. School and law enforcement say some security measures in place helped slow him down and may have prevented further bloodshed.

Silver Rail Elementary’s exterior fence — which alerted drivers to the shooter before he entered the school — and the school’s locked doors all helped to slow the shooter’s entry to the school, said officials. Human intervention also slowed and ultimately stopped the shooter, as a teacher in the classroom threw a cell phone at him and fought with him and a Samaritan followed him into the school and killed him.

The teenage gunman shot his way into the school Thursday afternoon, killing three children and a teacher and wounding two others before being shot himself by a parent.

The shooter arrived on foot to the school, approaching from an undeveloped lot across the street. He had a visible weapon.…