Audrey Hale, the 28-year-old Nashville school shooter fired 152 rounds during her 14-minute assault on the Covenant School that left six dead.

The Metropolitan Nashville Police Department revealed Monday that Hale reportedly planned the attack months in advance. The shocking details emerged in the latest update on the police investigation, which also disclosed that Hale plotted the massacre for a month in writings found inside their car and home.

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“[Hale] documented, in journals, [their] planning over a period of months to commit mass murder at The Covenant School,” police said in a press release Monday.

Police said Hale’s writings have been sent to the FBI’s Behavioral Analysis Unit in Quantico, Virginia. It is working with local investigators to figure out what drove Hale to slaughter three children and three staff members at their former primary school.

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According to police, a precise motive hasn’t been discovered but Hale “considered the actions of other mass murderers.”

Hale’s motive behind the shooting has been a mystery from the start.

Nashville Police Chief John Drake earlier speculated that Hale held “resentment” against former teachers of the school. Police also revealed that Hale was disturbed by the recent death of a close friend and was under the care of a doctor for an emotional disorder. She had sent a series of dark messages to a friend just before the assault started.

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On March 27, the day of the shooting, Drake said Hale left behind a “manifesto” that was being examined by detectives, but its contents haven’t been made public.

Cops said the barrage of bullets fired by Hale came from two assault rifles and a handgun. The three guns were part of a seven-gun arsenal Hale had legally gathered behind their parents’ backs. They stashed the guns throughout the Nashville home they shared, Drake said last week.

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Hale was shot and killed by two Nashville police officers around 14 minutes after they broke into the school by shooting through locked glass doors. The officers each fired four bullets, killing Hale at the scene.