A heavily armed 28-year-old fatally shot three children and three adult staffers on Monday at a private Christian school the suspect once attended in Tennessee's capital city before police killed the assailant, authorities said.

he motive was not immediately known, but the suspect had drawn detailed maps of the school, including entry points for the building, and left behind a "manifesto" and other writings that investigators were examining, Police Chief John Drake told reporters.

The latest in an epidemic of deadly mass gun violence that has come to routinely terrorize even the most cherished of U.S. institutions unfolded on a warm spring morning at The Covenant School, whose students consist mostly of elementary school-age children.

Drake identified the suspect as Audrey Elizabeth Hale, 28, a resident of the Nashville area, and referred to the assailant by female pronouns. The chief said the suspect identified as transgender but provided no further clarity.

Later, the Tennessean newspaper cited a police spokesperson as saying Hale used he/him pronouns. Hale used male pronouns on a LinkedIn page that listed recent jobs in graphic design and grocery delivery.

Addressing an early evening news conference, Drake said police were working on a theory about what may have precipitated the shooting and would "put that out as soon as we can." He said the suspect had no known prior criminal history.

In a subsequent NBC News television interview, Drake said investigators believed the shooting stemmed from "some resentment" the suspect harboured "for having to go to that school" as a younger person.

The police chief did not specify the nature of such presumed resentment, or whether it had anything to do with the suspect's gender identity or the Christian orientation of the school. Drake said the school was singled out for attack but the individual victims were targeted at random.

'SWIFT' POLICE RESPONSE

The Metropolitan Nashville Police Department began receiving calls at 10:13 a.m. of a shooter at the school, and arriving officers reported hearing gunfire coming from the building's second floor, police spokesperson Don Aaron told reporters.

Two officers from a five-member team shot the assailant in a lobby area, and the suspect was pronounced dead by 10:27 a.m.

"The police department response was swift," Aaron said.

The victims were identified as Evelyn Dieckhaus, Hallie Scruggs, and William Kinney, all age 9, along with staffers 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."

Reacting in Washington to the latest school shooting, U.S. President Joe Biden urged the U.S. Congress again to pass tougher gun reform legislation.

"It's sick," Biden said, addressing the issue during an event at the White House and urging Congress again to pass a ban on assault-style weapons. "We have to do more to stop gun violence. It's ripping our communities apart, ripping the soul of this nation."

U.S. Senator Marsha Blackburn, a Tennessee Republican, said on Twitter that her office stands "ready to assist" those affected by the shooting.