Nashville school shooter Audrey Hale spent months studying and planning to “commit mass murder”, police have said.

The lone attacker fired more than 150 rounds at the Covenant School on March 27 – armed with armed with two assault rifles, a handgun and “significant ammunition”.

Three nine-year-old students – Evelyn Dieckhaus, Hallie Scruggs and William Kinney – were killed during the massacre at the private Christian school in Nashville, Tennessee.

Staff members Katherine Koonce, Cynthia Peak and Mike Hill were also murdered before police officers killed Hale.

In details released by Metro Nashville Police a week on from the incident, investigators said they found detailed plans in journals found in Hale’s bedroom and car.

Police are yet to establish a motive for Hale, 28, a former Covenant School student who identified as a transgender man.

Investigators said the writings in Hale’s journals will “remain under careful review by the MNPD and by the FBI’s Behavioural Analysis Unit – based in Quantico, Virginia”.

They added: “The motive for Hale’s actions has not been established and remains under investigation by the Homicide Unit in consultation with the FBI’s Behavioral Analysis Unit.

“It is known that Hale considered the actions of other mass murderers.

“The investigation shows that Hale fired a total of 152 rounds (126 5.56 rifle rounds and 26 nine millimeter rounds) from the time she shot her way into the school until she was killed by police.”

Last week MNPD chief John Drake said Hale was under the care of a doctor for an “emotional disorder”.

He said police found seven firearm’s at Hale’s home, adding that his parents thought he only had one gun which he had sold.

The police chief also revealed that a map of the school grounds and a “manifesto” had been found at Hale’s home.

He told a press conference: “There’s right now a theory that we may be able to talk about later but it’s not confirmed, and so we’ll put that out as soon as we can.”