NASHVILLE, Tenn. - A former student shot through the doors of a Christian elementary school Monday and killed three children and three adults after elaborately planning the massacre by drawing out a detailed map and conducting surveillance of the building, police said.

The massacre at The Covenant School in Nashville was the latest in a series of mass shootings in a country that has grown increasingly unnerved by bloodshed in schools.

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 planned vigils for the victims.

“I was literally moved to tears to see this and the kids as they were being ushered out of the building,” Metropolitan Nashville Police Chief John Drake said during one of several news conferences.

Police gave unclear information on the gender of the shooter, who police say was fatally shot by two responding officers at the school. For hours, police identified the shooter as a 28-year-old woman and eventually identified the person as Audrey Hale. Then at a late afternoon news 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.

Drake did not give a specific motive when asked by reporters but gave chilling examples of the shooter’s prior planning for the targeted attack.

“We have a manifesto, we have some writings that we’re going over that pertain to this date, the actual incident,” he said. “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 Hale had “some resentment for having to go to that school.”

The shooter gained entry by firing into glass doors on the building, shattering them, police later said in a tweet.

The shooter was armed with two “assault-style” weapons as well as a handgun, authorities said. 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 the shooter’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.

Students held hands as they walked to school buses, which drove them to a nearby church to be reunited with their parents.

Rachel Dibble, who was at the church as families found their children, described the scene as everyone being in “complete shock.”

“People were involuntarily trembling,” said Dibble, whose children attend a different private school in Nashville. “The children … started their morning in their cute little uniforms, they probably had some Froot Loops and now their whole lives changed today.”

Communities around the U.S. has suffered through one mass killing after another in recent years, with school shootings taking an especially painful toll.

Recent tragedies nationwide include the massacre at an elementary school in Uvalde, Texas, last year; a first grader who shot his teacher in Virginia; and a shooting last week in Denver that wounded two administrators.

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.

“It’s ripping at the soul of this nation, ripping at the very soul of this nation,” Biden said.

Biden later ordered the U.S. flag to be flown at half-staff on all federal buildings through March 31. He also spoke to Tennessee Gov. Bill Lee and Nashville Mayor John Cooper about the shooting, officials said.

Founded as a ministry of Covenant Presbyterian Church - which is affiliated with the conservative evangelical Presbyterian Church in America - 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.