Investigators are searching for clues to better understand the motive behind the latest school shooting in the United States, which left three children and three adults dead at a private Christian primary school in Nashville, Tennessee.

Police on Tuesday were poring over what they described as a “manifesto” left by the attacker following the mass shooting at the Covenant School in the city’s leafy Green Hills community.

They identified the assailant as 28-year-old former student Audrey Elizabeth Hale.

Authorities described the attacker as a woman, although a police official said she “identified” as transgender, without elaborating.

Among the other evidence being assessed on Tuesday were writings by Hale and a detailed, hand-drawn map of the school showing various entry points, Metro Nashville Police Chief John Drake told reporters.

He told NBC News the evidence “indicates there was going to be shootings at multiple locations and that the school was one of them”. Drake added that the school appeared to be specifically targeted, but the individual victims were targeted at random.

“There’s some belief that there was some resentment for having to go to that school,” the police chief said.

Meanwhile, former schoolmate Averianna Patton told CNN that Hale wrote on Instagram on the morning of the shooting.

“One day, this will make more sense,” she said Hale wrote. “I’ve left behind more than enough evidence behind. But something bad is about to happen.”

Patton said she called the police to alert them at around the time the attack started.

The adults who were fatally shot were identified as Mike Hill, 61, a school custodian; Cynthia Peak, 61, a substitute teacher; and Katherine Koonce, 60, who was listed on the Covenant website as “head of school”.

Footage released

Surveillance footage released by police showed Hale wearing camouflage pants, a black vest over a T-shirt, and a backward red baseball cap. The assailant is seen shooting through a glass door to gain entry to the school before stalking the halls.

Police body camera footage showed law enforcement moving through the building before confronting and fatally shooting Hale.

The six minutes of footage, edited together from the body-worn cameras of two responding officers, starts with an officer retrieving a rifle from his trunk as a staff member tells him that the school is locked down but that two children are unaccounted for.

“Let’s go! I need three!” the officer yells as he enters the building, where alarms can be heard ringing. The video shows officers passing by bulletin boards and cubbies as they clear one room after another before heading upstairs.

Amid the sound of gunfire, the officers race down the hallway and into a lounge area, where the suspect is seen dropping to the floor after being shot.

Authorities have said Hale was carrying two assault-style weapons, including a rifle, as well a pistol. Those three guns were among seven Hale bought legally from five area stores, said Drake, the police chief.

The suspect’s parents did not know that Hale was in possession of seven guns, Drake told reporters, adding that they were under the impression that the attacker had owned only one gun but had sold it.

He said it appeared that the suspect also had some sort of training with firearms.

The Covenant School, founded in 2001, is a ministry of Covenant Presbyterian Church and has about 200 students, according to the school’s website.

The school serves preschool through sixth graders and held an active-shooter training programme in 2022, local television station WTVF-TV reported.

US school shootings

The attack marked the 90th school shooting – defined as any incident in which a gun is discharged on school property – in the United States this year, according to the K-12 School Shooting Database, a website founded by researcher David Riedman.

There were 303 such incidents in the US in 2022, according to the database, representing the highest annual tally since reporting began in 1970.

Since 2020, the number of mass shootings every year in the US has hovered above 600, with 646 recorded in 2022, according to the Gun Violence Archive database.

As of Tuesday, there had been 130 mass shootings in the US so far this year, according to the tracker, which considers a mass shooting any gun attack with four or more victims.

President Joe Biden has described the Nashville school shooting as “sick” and said gun violence was “ripping the soul of this nation” as he urged Congress to pass a federal ban on the assault weapons often used in mass shootings.

However, despite regular, high-profile attacks involving assault-style firearms, which are military-style weapons that can be fired rapidly, a federal ban has been a political non-starter.

“They need to act,” White House Press Secretary Karine Jean-Pierre said on Monday, referring to US lawmakers. “The time is now.”