"Today is a sad day not only for Madison, for our entire country," Chief Barnes said. "We have to do a better job in our community."
He added the police had not identified a motive in the shooting, and the suspect's family was co-operating with the investigation.
He named the alleged attacker as Natalie Rupnow - who also went by the name Samantha - and is believed to have died from a self-inflicted gunshot wound.
Chief Barnes said a second grade student was the first to call in the attack.
Officers responded to a 911 call of an active shooter at the Christian school around 11:00 local time (17:00GMT) on Monday. The attacker attended school before the shooting, Chief Barnes said.
The shooting was confined to a study hall with students in mixed grades.
Barbara Wiers, director of relations at the school, said the school had conducted active shooter training earlier this year and the information was "very fresh" for educators to put into practice on Monday.
She said while the school does not have a dedicated police officer, known as a school resource officer, the doors of all classrooms automatically lock and anyone wanting to gain entry to the campus must be buzzed in through the primary entrance.
Ms Wiers, who said she was teaching at the time of the attack, said students handled themselves "brilliantly".
"They were clearly scared," she said. "When they heard 'lockdown, lockdown' and nothing else, they knew it was real."
Police say they found the shooter dead when they arrived at the school, along with a handgun. No officers fired weapons.
Police have not named any of the victims.
Chief Barnes said two students were in critical condition in the hospital facing life-threatening injuries. Four others were taken to hospital with non life-threatening injuries, two of whom have since been released.
Authorities have appealed for witnesses who saw or heard the attack to come speak to police, and that they hope these accounts will shed light on the attacker's motive.
"But that's not something we want to rush. We're not gonna interrogate students," Chief Barnes said. "We're gonna give them an opportunity to come in and tell us what they saw when they're ready."
He added that "ever child, every person in that building is a victim and will be a victim forever".
The chief said officers responded to the school as they were undergoing training at a training centre for law enforcement located three miles away.
"What began as a training day became an actual day," he said.
The shooting also resulted in a large response from emergency officials. Madison Fire Chief Chris Carbon said 15 ambulances respond.
Officials from the FBI also responded, as well as other federal and local law enforcement officials.
The Abundant Life Christian School has around 400 students ranging from kindergarten through high school.
"Please pray for our Challenger Family," the school wrote in a post on Facebook. The post quickly received hundreds of comments of support from people across the US.
Wisconsin Governor Tony Evers said that he was closely monitoring the situation and praying for everyone involved. He also ordered that flags fly at half mast on state buildings.
President Joe Biden said in a statement that the shooting was "shocking and unconscionable".
"Students across our country should be learning how to read and write – not having to learn how to duck and cover," said Biden, who also called on Congress to act immediately on legislation that could prevent more gun violence.
Shootings are common in the US, and schools are no exception.
The K-12 Violence Project, a non-profit working on reducing violence through accessible and actionable research, has counted more than 300 shootings in 2024. These include events where a gun is brandished or fired, or a bullet hits school property for any reason, regardless of the number of victims.
According to the news organisation EducationWeek, 38 school shootings have resulted in deaths or injuries across the US this year. There were a total of 69 victims - including 16 deaths - before today's shooting.
Mass shootings by females are far less common, however. School shootings committed by female attackers are even less common.
In a blog post last year, K-12 School Shooting Database founder David Riedman wrote that the vast majority of school shooters are males in their teens or early 20's. However, at least four planned school shootings were by female attackers dating back to 1979.
1
2
3
4
.svg)
