Shon Barnes, the police chief in Madison said investigators were aware of a “manifesto, if you want to call it that, or some type of letter” posted by someone who might have known the shooter, 15-year-old student Natalie Rupnow.

“We haven’t been able to locate that person yet, but that’s something we’re going to work on today,” Barnes told CNN. “We’ll also be looking through (Rupnow’s) effects, if she had a computer or cell phone, to see if there are any transmissions between her and someone else, and that will give us an idea of what type planning.”

Police responding to the shooting Monday at Abundant Life Christian School found Rupnow had a self-inflicted gunshot wound. The shooter died on the way to a hospital.

Police said the shooting occurred in a study hall. Besides the deaths, six people were wounded, including two students who remain in critical condition.

Abundant Life is a nondenominational Christian school — prekindergarten through high school — with approximately 420 students in Madison, the state capital.

Barbara Wiers, the school’s director of elementary and school relations, said when they practice safety routines, leaders always announce that it’s a drill. That didn’t happen Monday, the last week before Christmas break.

“When they heard, ‘lockdown, lockdown,’ they knew it was real,” she said.

Wiers said the school does not have metal detectors but uses cameras and other security measures.

Barnes said police were talking with the shooter’s father and other family members, who were cooperating, and searching the shooter’s home.

“He lost someone as well,” Barnes said of the father. “And so we’re not going to rush the information. We’ll take our time and make sure we do our due diligence.”

The shooter’s parents, who are divorced, jointly shared custody of their child, but the shooter primarily lived with her 42-year-old father, according to court documents.

At the school, the first 911 call to report an active shooter came in shortly before 11 a.m. Monday. First responders who were in training just 3 miles (about 5 kilometers) away dashed to the school for an actual emergency, Barnes said. They arrived three minutes after the initial call.

Investigators believe the shooter used a 9mm pistol, a law enforcement official told The Associated Press. The official spoke on the condition of anonymity because they were not authorized to discuss the ongoing investigation.

Children and families were reunited at a health clinic about a mile (1.6 kilometers) from the school. Parents pressed children against their chests while others squeezed hands and shoulders as they walked side by side.

Abundant Life asked for prayers in a brief Facebook post. Wiers said they’re still deciding whether they will resume classes this week.

Bethany Highman, the mother of a student, rushed to the school and learned over FaceTime that her daughter was OK.

“As soon as it happened, your world stops for a minute. Nothing else matters,” Highman said. “There’s nobody around you. You just bolt for the door and try to do everything you can as a parent to be with your kids.”

In a statement, President Joe Biden cited the tragedy in calling on Congress to pass universal background checks, a national red flag law and certain gun restrictions.

“We can never accept senseless violence that traumatizes children, their families, and tears entire communities apart,” Biden said. He spoke with Wisconsin Gov. Tony Evers and Madison Mayor Satya Rhodes-Conway and offered his support.

Evers said it’s “unthinkable” that a child or teacher would go to school and never return home.

The school shooting was the latest among dozens across the U.S. in recent years, including especially deadly ones in Newtown, Connecticut; Parkland, Florida; and Uvalde, Texas.

The shootings have set off fervent debates about gun control and frayed the nerves of parents whose children are growing up accustomed to doing active shooter drills in their classrooms. But school shootings have done little to move the needle on national gun laws.

Firearms were the leading cause of death among children in 2020 and 2021, according to KFF, a nonprofit that researches health care issues.

Rhodes-Conway said the country needs to do more to prevent gun violence.

“I hoped that this day would never come to Madison,” she said.