A female shooter killed three children and three adults at a Christian grade school Monday in the southern U.S. state of Tennessee, according to authorities.

Police shot and killed the suspected shooter, who they initially said was a teenager. Authorities later determined the suspect was 28 years old.

A spokesperson for the Metropolitan Nashville Police Department, Don Aaron, said the shooter was armed with at least two semi-automatic rifles and a handgun.

He said police began receiving calls at 10:13 a.m. about a shooter at The Covenant School, a private Presbyterian school in Nashville.

Officers who arrived at the scene could hear gunfire coming from the school's second floor, according to Aaron.

He said two officers who were part of a five-member team shot the shooter in a lobby area, and she was dead by 10:27 a.m.

'We do not know who she is at this juncture,' Aaron said.

Police described the victims as three students and three adult staff members. They said besides those six people, no one else was shot.

Police escorted the other students to a nearby church, where parents were told to gather.

The Covenant School has about 200 students from pre-school to sixth grade, according to its website.

"In a tragic morning, Nashville joined the dreaded, long list of communities to experience a school shooting," Nashville Mayor John Cooper tweeted.

"My heart goes out to the families of the victims. Our entire city stands with you," he wrote.

While mass shootings are becoming more commonplace in the United States, female shooters are rare.

Of the 187 mass shootings since 1996, only four were carried out by women, according to The Violence Project, a nonprofit group that tracks mass shootings.

Last year, a mass shooting at an elementary school in Uvalde, Texas left 21 people dead.

Some information in this report came from The Associated Press and Reuters.