The former student who shot through the doors of a Christian elementary school in Nashville and killed three children and three adults had drawn a detailed map of the school, including potential entry points, and conducted surveillance of the building before carrying out the massacre.

Metropolitan Nashville Police Chief John Drake did not say exactly what drove the shooter to open fire Monday morning at The Covenant School before being killed by police. But he provided chilling examples of the shooter’s elaborate planning for the targeted attack, the latest in a series of mass shootings in a country that has grown increasingly unnerved by bloodshed in schools.