The person who killed three children and three adults at a Christian school in Nashville legally bought seven firearms in recent years and hid the guns from their parents before the attack, police said on Tuesday.

The violence on Monday at The Covenant School is the latest school shooting to roil the nation. Three 9-year-old students were killed, as well as the head of the grade school, a custodian and a substitute teacher.

The suspect, Audrey Hale, 28, was a former student at the school. Police said the shooter did not target specific victims.

Authorities said Hale was not on their radar before the attack. Police say Hale was under a doctor's care for an undisclosed emotional disorder.

Police have released videos of the shooting, including edited surveillance footage that shows the shooter's car driving up to the school, glass doors being shot out and the shooter ducking through one of them.

Additional video, from Officer Rex Engelbert's bodycam, shows a woman greeting police outside as they arrive at The Covenant School on Monday. “The kids are all locked down, but we have two kids that we don't know where they are,” she tells police.

“OK, yes, ma'am,” Engelbert replies.

The woman then directs officers to Fellowship Hall and says people inside had just heard gunshots. “Upstairs are a bunch of kids,” she says.

Three officers, including Engelbert, search rooms one by one, holding rifles. “Metro Police,” officers yell.

“Let's go, let's go,” one officer yells.

As alarms are heard going off in the school, one officer says, “It sounds like it's upstairs.”

Officers climb stairs to the second floor and enter a lobby area. “Move in,” an officer yells. Then a barrage of gunfire is heard.

“Get your hands away from the gun," an officer yells twice. Then the shooter is shown motionless on the floor.

Police earlier said Hale had drawn a detailed map of the school, including potential entry points, and conducted surveillance of the building before carrying out the massacre, authorities said.

Police response times to school shootings have come under greater scrutiny after the elementary school massacre in Uvalde, Texas, in which 70 minutes passed before law enforcement stormed the classroom.

In Nashville, police have said 14 minutes passed from the initial call about a shooter in the school to when the suspect was killed, but they have not said how long it took them to arrive.

Surveillance video of The Covenant School grounds released by police shows a time stamp of just before 10.11 am, when the glass doors were shot out by the shooter. Police said they received a call about a shooter at 10.13 am but have not said precisely what time they arrived, and the edited bodycam footage didn't include time stamps.

A police spokesperson didn't immediately respond to an email Tuesday asking when they arrived or whether any version of the video includes time stamps.

Police have given unclear information on Hale's gender. For hours Monday, police identified the shooter as a woman. At a late afternoon press conference, the police chief said Hale was transgender. After the news conference, police spokesperson Don Aaron declined to elaborate on how Hale identified.

In an email on Tuesday, police spokesperson Kristin Mumford said Hale “was assigned female at birth. Hale did use male pronouns on a social media profile.”

The victims were children Evelyn Dieckhaus, Hallie Scruggs and William Kinney, all age 9. The adults were 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.

Metropolitan Nashville Police Chief John Drake did not say exactly what drove Hale but said in an interview with NBC News that investigators believe the shooter had “some resentment for having to go to that school.”

Drake 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.”

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.

President Joe Biden said he had spoken to the Nashville chief of police, mayor and senators in Tennessee. He pleaded with Congress to pass stronger gun safety laws, including a ban on assault weapons.

“The Congress has to act," Biden said. The majority of the American people think having assault weapons is bizarre, it's a crazy idea. They're against that.”

Founded as a ministry of Covenant Presbyterian Church, The Covenant School is in the affluent Green Hills neighbourhood just south of downtown Nashville that is home to the famous Bluebird Cafe, beloved by musicians and songwriters.

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

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.

At The Covenant School, officers began clearing the first story 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.

Police identified Engelbert, a four-year member of the force, and Collazo, a nine-year member, as the officers who fatally shot Hale.

The surveillance video released Monday shows 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 labelled “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.?