A shooter opened fire Wednesday morning on children celebrating mass during the beginning of their school day at a Catholic church in Minneapolis, police said, leaving three people dead, including the shooter, and 19 injured.

The children killed were eight and 10 years old, Minneapolis police Chief Brian O'Hara said at a news conference of local officials. O'Hara said the shooter set up outside a window of Annunciation Church and fired into it, striking worshippers in pews.

The shooter was armed with a rifle, shotgun and pistol, he said. In a later update Wednesday afternoon, officials said the weapons had been purchased legally.

"The sheer cruelty and cowardice, firing into a church full of children, is absolutely incomprehensible," O'Hara said.

There was no indication the shooter, believed to be in their early 20s, had any affiliation with Annunciation Catholic School, officials said.

Speaking hours later, Minnesota Gov. Tim Walz said the children inside the church were "met with evil and horror, and death."

"We often come to these unspeakable tragedies and say, 'There are no words for this.' There shouldn't be words for this because they shouldn't happen," he said.

The governor called the shooting "unthinkable, but it's all too common, not just in Minnesota, but across this country."

The shooting is believed to have been isolated and not connected with any other violent incidents in the city. One of those incidents, which happened Tuesday afternoon, saw one person killed and six others hurt in a shooting outside a high school in Minneapolis.

In the afternoon update, police identified the assailant as 23-year-old Robin Westman, who had no known criminal history.

Minneapolis city officials had said earlier that the shooter had been "contained" after the gunfire, and there was no longer any "active threat" to residents. O'Hara said it was believed the shooter died from a self-inflicted gunshot.

Among the injured were 14 children and three adults in their 80s. According to Hennepin Healthcare, the hospital treating victims of the shooting, one adult and six children are in critical condition, and one adult and two children are being treated for non-life threatening injuries. In his afternoon update, O'Hara said all of these victims are expected to survive.

O'Hara also said dozens of officers responded to the shooting and many of them — as well as the children and staff present in the church — are deeply traumatized by what they saw.

Clarissa Garcia, a student at the school who witnessed the shooting, told local reporters she was led downstairs by a teacher to hide in a preschool classroom.

"I was just at church and I heard something really loud. Like, I thought it was fireworks in the church, and then I saw the shooting and I was like, 'Oh my gosh, I'm so scared,'" Clarissa said, her mother Suzanne Garcia at her side.

Once inside the downstairs classroom, Clarissa said she and a friend "were just praying and praying."

"You cannot put into words the gravity, the tragedy or the absolute pain of this situation," Minneapolis Mayor Jacob Frey said.

Frey and Annunciation's principal said teachers and children, too, responded heroically.

"Children were ducked down. Adults were protecting children. Older children were protecting younger children," said the principal, Matt DeBoer.

The school was evacuated, and students' families were later directed to a "reunification zone" at the school. Outside, amid a heavy uniformed law enforcement presence, were uniformed children in their dark green shirts or dresses. Many were trickling out of the school with adults, giving lingering hugs and wiping away tears.

Danielle Gunter, the mother of an eighth-grade boy who was shot, said in a statement that her son told her a Minneapolis police officer "really helped him" by giving aid and a hug before her son got into an ambulance.