NASHVILLE, Tennessee - Video released on Tuesday showed Nashville, Tennessee, police officers storming a private Christian grade school on Monday before confronting and fatally shooting an attacker who killed three nine-year-old students and three adult staff members.

The six minutes of harrowing footage, edited together from the body-worn cameras of two responding officers and released by the Metropolitan Nashville Police Department, showed officers clearing several first-floor classrooms before heading upstairs to the second floor as gunfire is heard.

The officers run down a hallway - past what appears to be a victim lying on the ground - and into a lounge area, where the suspect is seen dropping to the floor after being shot.

Police have identified the shooter as Audrey Elizabeth Hale, a 28-year-old former student at the Covenant School. Investigators are examining what they called a “manifesto” that Hale left behind, hoping to learn what motivated the latest in a long string of US mass shootings.

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

Last year saw 303 such incidents, the highest of any year in the database, which goes back to 1970.

During a previous press briefing, Metro Nashville police chief John Drake said Hale self-identified as transgender, although Mr Drake offered no further clarity. The police chief and other officials repeatedly referred to the suspect with female pronouns, though Hale used male pronouns on a LinkedIn page that listed recent jobs in graphic design and grocery delivery.

Mr Drake said Hale had a detailed, hand-drawn map of the school showing various entry points. Hale was armed with two assault-style weapons, one of them a rifle, as well as a 9mm pistol, authorities said.

The police chief told NBC the manifesto indicated that Hale planned to carry out additional shootings at other locations. He said the Covenant School was singled out for attack but that the individual victims were targeted at random.

Investigators believe Hale harboured “some resentment for having to go to that school” as a child, Mr Drake said, without elaborating.

The three slain children were identified as Evelyn Dieckhaus, Hallie Scruggs and William Kinney. Also shot dead were Katherine Koonce, 60, the head of school; Mike Hill, 61, a school custodian; and Cynthia Peak, 61, a substitute teacher.

Scruggs’ father, Chad, is a pastor at the Covenant Presbyterian Church, which is connected to the school. In a statement given to ABC News, he said the family was heartbroken.

“Through tears we trust that she is in the arms of Jesus who will raise her to life once again,” the statement read.

‘Get your hands away from the gun’

Nashville police began receiving calls about a shooter at 10.13am, spokesman Don Aaron told reporters. The suspect was pronounced dead by 10.27am.

“The police department response was swift,” Mr Aaron said.

The body camera footage showed officers rapidly searching for the shooter, in contrast to videos showing officers in Uvalde, Texas, waiting inside Robb Elementary School for more than an hour last May as a gunman inside a classroom continued an attack that claimed the lives of 19 children and two adults.

In 2018, a number of officers responding to a school shooting in Parkland, Florida, remained outside the building rather than immediately pursuing the suspect, according to a state commission’s investigation. Seventeen students and staff members died in that attack.

Both incidents prompted fresh scrutiny of police protocols for active shooters, which call for officers to engage the suspect at once to prevent loss of life.

The start of the Nashville police video shows 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 uses a key to unlock a door and enter 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. When the officers reach the second floor, one says, “We’ve got one down,” before they race down the hallway to confront the shooter.

Officer Rex Engelbert and Officer Michael Collazo - whose body cameras provided the footage - both fire several rounds at the suspect. The video shows the assailant still moving on the ground as another officer repeatedly yells, “Get your hands away from the gun!”

Police previously released surveillance footage showing the shooter arriving at the school and gaining entry by shooting through a glass door. The suspect is seen stalking through empty hallways as emergency lights flash, brandishing a rifle and entering rooms, seemingly looking for people.

The Covenant School, founded in 2001, serves about 200 students from preschool to sixth grade in the Green Hills neighbourhood of Tennessee’s state capital, according to the school’s website. REUTERS