CNN– The 28-year-old who killed three children and three adults at a Christian school in Nashville last week fired 152 rounds, Nashville police said Monday, adding that while they have yet to determine a motive, the shooter planned the attack “over a period of months” and studied other mass murderers.

The shooter, identified as former student Audrey Hale, “acted totally alone,” Metro Nashville Police said in the Monday update. Investigators with the police department and the FBI’s Behavioral Analysis Unit continue to review writings left behind by Hale, which police said Monday contain documentation of Hale’s planning.

“It is known that Hale considered the actions of other mass murderers,” the release said.

Most of the rounds fired by Hale – 126 – were 5.56 rifle rounds, and 26 were 9 mm rounds, police said.

Monday’s update on the investigation comes a week after Hale – armed with three firearms, according to police – stormed the Covenant School, gaining entry to the building by firing through the building’s locked glass doors.

Within minutes, Nashville officers arrived at the scene and confronted Hale as the shooter was “firing through a window at arriving police cars,” per police. Two officers opened fire – a moment captured in bodycam footage later released by police – and killed Hale at 10:27 a.m., 14 minutes after the shooter entered, according to Nashville police spokesperson Don Aaron.

Officer Rex Engelbert fired four 5.56 rounds from his rifle, police said Monday, while Officer Michael Collazo fired four rounds from his 9 mm pistol.

Evelyn Dieckhaus, William Kinney and Hallie Scruggs, all 9 years old, were killed in the massacre, as well as school custodian Mike Hill, 61, substitute teacher Cynthia Peak, 61, and Katherine Koonce, 60, who was head of the school.

While police are still working to determine a motive for the attack, they said in a news release that writings left behind by Hale make clear that it was “calculated and planned.” Hale targeted the school and Covenant Presbyterian Church, to which the school is attached, police said, but it’s believed the victims were fired upon at random.

The school shooting – the deadliest since 21 people, including 19 children, were killed at a school in Uvalde, Texas, last May – renewed the debates over the scourge of American gun violence, access to firearms and school safety.

Hale, who police said was under care for an emotional disorder, had legally purchased seven firearms and hidden them at home, per Metropolitan Nashville Police Chief John Drake.