Police on Tuesday were working to find a motive behind a deadly rampage that left three primary school pupils and three staff dead.
The perpetrator, a former student, killed the schoolchildren in what police believe was a methodically planned ambush.
Nashville Police Chief John Drake provided disturbing details of the shooter’s intricate planning for the attack at the Christian Covenant School, a small private school of 200 pupils.
“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."
Mr Drake told NBC the manifesto "indicates there was going to be shootings at multiple locations and that the school was one of them”.
He said the school was singled out for attack but that the victims were targeted at random, and believed the shooter had “some resentment for having to go to that school".
Police have named the shooter as Audrey Hale, saying the 28-year-old was born female but identified as transgender.
Former schoolmate Averianna Patton described to CNN a missive that Hale allegedly posted on Instagram on the morning of the shooting.
"One day this will make more sense," it read.
"I've left behind more than enough evidence behind. But something bad is about to happen."
After reading the post, Ms Patton said she called to warn police at about the same time the attack started.
Alarming school surveillance video released by authorities shows the shooter breaching the structure’s glass doors with gunfire from a semi-automatic weapon.
The video shows the assailant armed with two assault rifles and a handgun stalking the empty halls of the eerily quiet school, opening random doors in an apparent search for victims.
WARNING: Police surveillance footage that may be disturbing
Surveillance footage shows Nashville school shooter enter building
The shooter appears to wear a black military-style vest, camouflage pants and red baseball cap. Officers' bodycam footage shows police ultimately confronting and then shooting the suspect.
The three schoolchildren killed, all aged 9, were Evelyn Dieckhaus, Hallie Scruggs and William Kinney. Also shot dead were Mike Hill, 61, a school custodian; Cynthia Peak, 61, a substitute teacher; and Katherine Koonce, 60, listed on the Covenant website as head of school.
Police said at least two of the weapons were purchased legally and that Hale had several rounds of ammunition and was "prepared for confrontation with law enforcement".
President Joe Biden described the latest shooting as "sick" and said gun violence was "ripping the soul of this nation", as he urged Congress to pass a ban on the assault weapons often used in mass shootings.
Agencies contributed to this report
