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.

“We have a manifesto, we have some writings that we’re going over that pertain to this date, the actual incident,” he told reporters. “We have a map drawn out of how this was all going to take place.”

He said in an interview with NBC News that investigators believe the shooter had “some resentment for having to go to that school.”

The victims included three 9-year-old children, the school’s top administrator, a substitute teacher and a custodian. Amid the chaos a familiar ritual played out: Panicked parents rushed to the school to see if their children were safe and tearfully hugged their kids, and a stunned community held vigils for the victims.

Rachel Dibble, who was at a nearby church where children were taken to be reunited with their parents, described the scene as everyone being in “complete shock.”

“People were involuntarily trembling,” she said. “The children … started their morning in their cute little uniforms, they probably had some Froot Loops and now their whole lives changed today.”

Police gave unclear information on the gender of the shooter. For hours, police identified the shooter as a 28-year-old woman and eventually identified the person as Audrey Elizabeth Hale. Then at a late afternoon press conference, the police chief said that Hale was transgender. After the news conference, police spokesperson Don Aaron declined to elaborate on how Hale currently identified.

Authorities said Hale was armed with two “assault-style” weapons as well as a handgun. At least two of them were believed to have been obtained legally in the Nashville area, according to the chief. Police said a search of Hale’s home turned up a sawed-off shotgun, a second shotgun and other unspecified evidence.

The victims were identified as Evelyn Dieckhaus, Hallie Scruggs, and William Kinney, all 9 years old, and adults Cynthia Peak, 61; Katherine Koonce, 60; and Mike Hill, 61.

The website of The Covenant School, a Presbyterian school founded in 2001, lists a Katherine Koonce as the head of the school. Her LinkedIn profile says she has led the school since July 2016. Peak was a substitute teacher and Hill was a custodian, according to investigators.

Founded as a ministry of Covenant Presbyterian Church, The Covenant School is located in the affluent Green Hills neighborhood just south of downtown Nashville that is home to the famed Bluebird Café – a spot typically beloved by musicians and songwriters.

The school has about 200 students from preschool through sixth grade, as well as roughly 50 staff members.

“Our community is heartbroken,” a statement from the school said. “We are grieving tremendous loss and are in shock coming out of the terror that shattered our school and church. We are focused on loving our students, our families, our faculty and staff and beginning the process of healing.”

Before Monday’s violence in Nashville, there had been seven mass killings at K-12 schools since 2006 in which four or more people were killed within a 24-hour period, according to a database maintained by The Associated Press and USA Today in partnership with Northeastern University. In all of them, the shooters were males.

The database does not include school shootings in which fewer than four people were killed, which have become far more common in recent years. Just last week alone, for example, school shootings happened in Denver and the Dallas-area within two days of each other.

Monday’s tragedy unfolded over roughly 14 minutes. Police received the initial call about an active shooter at 10:13 a.m.

Officers began clearing the first story of the school when they heard gunshots coming from the second level, Aaron said. Police later said the shooter fired at arriving officers from a second-story window and had come armed with significant ammunition.

Two officers from a five-member team opened fire in response, killing the suspect at 10:27 a.m., Aaron said.

Late Monday night, police released approximately two minutes of edited surveillance video showing the shooter’s car driving up to the school from multiple angles, including one in which children can be seen playing on swings in the background. Next an interior view shows glass doors to the school being shot out and the shooter ducking through one of the shattered doors.

More footage from inside shows the shooter walking through a school corridor holding a gun with a long barrel and walking into a room labeled “church office,” then coming back out. In the final part of the footage, the shooter can be seen walking down another long corridor with the gun drawn. The shooter is not seen interacting with anyone else on the video, which has no sound.

Aaron said there were no police officers present or assigned to the school at the time of the shooting because it is a church-run school.

President Joe Biden, speaking at the White House on Monday, called the shooting a “family’s worst nightmare” and implored Congress again to pass a ban on certain semi-automatic weapons.

A reeling city mourned during multiple vigils Monday evening. At Belmont United Methodist Church, teary sniffling filled the background as vigil attendees sang, knelt in prayer and lit candles. They lamented the national cycle of violent and deadly shootings.

“We need to step back. We need to breathe. We need to grieve,” said Paul Purdue, the church’s senior pastor. “We need to remember. We need to make space for others who are grieving. We need to hear the cries of our neighbors.”