A gunman killed two young children and injured 17 people gathered at Annunciation Catholic Church in Minneapolis for a service on Wednesday to mark the first week of the parish's school, police said.
Fourteen of the injured are children, two of whom are in critical condition, Minneapolis Police Chief Brian O'Hara said at a news conference.
The children who died were 8 and 10 years old, O'Hara said.
The shooter, who officials later identified as Robin Westman, is dead from an apparently self-inflicted gunshot wound, he said.
Westman left behind multiple online videos referencing violent thoughts and suicide, as well as a sketch of a church's interior and an apology to their family, senior law enforcement officials briefed on the matter told NBC News.
The attack occurred just before 8:30 a.m. local time, as the children attended the service at the church.
The gunman, armed with a rifle, a pistol and a shotgun, and clad in black cargo-style pants, was positioned on the outside of the building and began firing through windows toward the children, who were sitting in pews.
The shooter, who is in his 20s and does not have an extensive criminal history, is believed to have acted alone, O'Hara said.
The chief said he was not aware if the shooter was a former employee or student of Annunciation.
"The sheer cruelty and cowardice, the firing into a church full of children, is absolutely incomprehensible," O'Hara said.
Minneapolis Mayor Jacob Frey said: "Don't just say this is about thoughts and prayers right now. These kids were literally praying!"
"There are no words that can capture the horror and the evil of this unspeakable act," Frey said.
"Do not think of these as just somebody else's kids. Those families are suffering immense pain right now. Think of this as if it were your own," the mayor added.
Victims have been admitted to two different hospitals. The nine children being treated at Hennepin Healthcare Hospital are between 6 and 14 years old, the hospital said at a news conference.
Before the shooter was declared contained, Gov. Tim Walz said that he had been briefed on the shooting and that he was "praying for our kids and teachers whose first week of school was marred by this horrific act of violence."
President Donald Trump spoke to Walz after the shooting, a White House official told NBC.
Trump "called to offer his condolences to the people of Minnesota and Governor Walz thanked him for the support," a source familiar with the call told NBC.
In a proclamation Wednesday afternoon, Trump ordered that American flags be flown at half mast at the White House and on all U.S. public grounds and military posts until Sunday at sunset.
Minnesota's U.S. senators, Amy Klobuchar and Tina Smith, thanked first responders and expressed grief over the violence.
"It's the first week of school. These kids should not be fearing for their lives," Smith wrote.
