A female shooter armed with two "assault-style" rifles and a pistol shot dead three adults and three nine-year-old children at a private school in Nashville, authorities said.
Emergency services were called to The Covenant School in the US city following reports of an "active shooter event" on Monday morning.
The gunwoman, 28, from Nashville, died after being shot by police.
Authorities were working to identify her and whether she had a connection to the church school.
The victims were pronounced dead after arriving at Monroe Carell Jr. Children's Hospital and Vanderbilt University Medical Center.
President Joe Biden called on Congress again to pass his assault weapons ban in the wake of the shooting.
"It's heartbreaking, a family's worst nightmare," he said.
The tragedy unfolded over roughly 14 minutes. At 10.13am on police received the initial call about an active shooter at the Presbyterian school, which teachers about 200 students aged three to 11.
Officers began clearing the first story of the school when they heard gunshots coming from the second level, police spokesperson Don Aaron said during a news briefing.
Two officers from a five-member team opened fire in response, fatally shooting the suspect at 10.27 am, Aaron said.
He said there were no police officers present or assigned to the school at the time of the shooting because it is a church-run school.
Other students walked to safety, holding hands as they left their school surrounded by police cars, to a nearby church to be reunited with their parents.
The school, founded as a ministry of Covenant Presbyterian Church in 2001, is located in the Green Hill neighborhood just south of downtown Nashville, situated close to the cities top universities.
Jozen Reodica heard the police sirens blaring from outside her office building nearby. As her building was placed under lockdown, she took out her phone and recorded the chaos.
"I thought I would just see this on TV," she told the Associated Press. "And right now, it's real."
The killings come as communities around the US reel from a spate of school violence, including the massacre at an elementary school in Uvalde, Texas, last year; a first grader who shot his teacher in Virginia; and a shooting last week in Denver that wounded two administrators.
“In a tragic morning, Nashville joined the dreaded, long list of communities to experience a school shooting,” Mayor John Cooper wrote on Twitter. “My heart goes out to the families of the victims. Our entire city stands with you.”
