Witnesses describe harrowing scenes at the hospital and school.

A gunman fired a rifle through the windows of a Catholic church in Minneapolis where students were celebrating their first Mass of the new school year on Wednesday, killing an 8-year-old and a 10-year-old in the pews and injuring 17 others, the police said.

The attacker then shot and killed himself in the rear of the church, the Minneapolis police chief, Brian O’Hara, said at a news conference. Of the 17 people injured, 14 were children, the chief said. Two were in critical condition, he said.

The shooting at a Catholic school in Minneapolis was the fourth deadly shooting in the city in just over 24 hours. In total, the string of violence has left at least five people dead and 25 injured, according to the police.

Police officials said the first shooting happened just before 1:30 p.m. on Tuesday, when a man stepped out of a vehicle near the Cristo Rey Jesuit High School in the Phillips neighborhood and fired about 30 rounds from a high-velocity .223 rifle at a group of people on a sidewalk. One person was killed and six others were injured.

One adult and six children remain in critical condition at Hennepin County Medical Center, according to the hospital. Two other children and one adult are now being treated for non-life-threatening injuries.

Minneapolis has seen a string of deadly violence in the past 24 hours. The shooting at Annunciation Catholic Church was the fourth in the city since Tuesday, and the second near a school. In total, the attacks have left at least five people dead and 25 injured, according to the police.

Ashlee Sherrill, a spokeswoman for the Bureau of Alcohol, Tobacco, Firearms and Explosives in Minnesota, said the agency had conducted an expedited review of the purchase history of the firearms used in the shooting. That information is now in the hands of investigators trying to determine whether the firearms were lawfully purchased.

Za’khia Jones, 29, was with her fiancée in the Hennepin County Medical Center emergency room when she saw children who appeared to be bloody being rushed in. “My heart was so heavy,” she said. “It was just a very sad situation.”

Patty Brown, 64, of McCook, Neb., said her daughter Kristi Forte was dropping off her 5-year-old son at a program at the school when the shooting occurred.

“My daughter was pretty upset,” she said, detailing how Ms. Forte told her that the gym “was full of crying parents and teachers.”

The Catholic church and school targeted in a shooting in Minneapolis on Wednesday was founded more than a century ago, and an all-school Mass during the first week of classes was an annual tradition.

Students from prekindergarten to eighth grade had started the new year on Monday at the facility, Annunciation Catholic School, and dozens of children and staff members were inside the Mass being celebrated at the church next door on Wednesday, the police said.

Two adults and nine children between the ages of 6 and 14 were treated at Hennepin County Medical Center, according to Dr. Thomas Wyatt, the chair of emergency medicine. Seven of the children were brought in as critical patients, and four required an operation. “We are dealing with gunshot wound injuries from a high-velocity weapon,” he said. He declined to expand on the injuries, but noted there have been no fatalities so far.

The all-school Mass during the first week of school is an annual tradition at Annunciation, and this morning, the pews would have been full of dozens of children in green-and-blue plaid uniforms, accompanied by teachers and staff members.

Officials did not identify the gunman but said he was in his early 20s.

Most of the shooting happened outside the building, and some doors of the church had been barricaded from the outside, Chief Brian O’Hara of the Minneapolis Police said. He estimated there were dozens of rounds fired from all three weapons — a rifle, shotgun and pistol. The police did not find any explosives, though a “firework that would release smoke” was found.

The gunman shot and killed himself in the rear of the church, according to the Minneapolis police chief.

Two children, ages 8 and 10, were killed in the pews in the shooting, the Minneapolis police chief said. Seventeen other people were injured, including 14 children. Two of those children are in critical condition.

The suspect shot into Annunciation Catholic Church from the outside during morning Mass, said Brian O’Hara, the Minneapolis police chief. He was armed with a rifle, shotgun and pistol.

Mayor Frey, who has led Minneapolis since 2018, has long been a vocal advocate for gun control and has devoted police resources to removing illegal guns from the streets.

“Children are dead, there are families that have a deceased child. You cannot put into words the gravity, tragedy or absolute pain of the situation,” Mayor Jacob Frey of Minneapolis said in a news conference on the shooting outside Annunciation Catholic School.

“Don’t just say this is about thoughts and prayers right now, these kids were literally praying,” he added. “It was the first week of school, they were in a church.”

Many victims are being treated at the Hennepin County Medical Center in downtown Minneapolis, according to officials. The extent of injuries is unknown. There are now six victims being treated at Children’s Minnesota, a hospital that has a trauma one center, according to that hospital’s statement.

Katie Cashman, a Minneapolis City Council member, said in a statement that “multiple children and staff” were shot and wounded while attending morning Mass.

President Trump just spoke with Gov. Tim Walz of Minnesota by phone, a White House official said.

The shooting left at least three dead, including the gunman, and 20 injured, according to two law enforcement officials who spoke on the condition of anonymity because the investigation was ongoing. They cautioned that the death toll was likely to rise.

Minnesota officials, including Mayor Jacob Frey of Minneapolis, are expected to hold a news conference shortly to provide information on the shooting.

The Children’s Minnesota hospital said it was treating five children after the shooting, but it would not comment on the extent of their injuries or their exact ages.

The shooter at a Minneapolis school has been “contained,” the City of Minneapolis said on social media. “There is no active threat to the community at this time,” the statement said.

The Bureau of Alcohol, Tobacco, Firearms and Explosives said on social media that its agents were at Annunciation Catholic Church in South Minneapolis, which has a school for children from prekindergarten through eighth grade. The church’s website indicates that students started the school year on Monday and that an all-school Mass had been scheduled for Wednesday morning.

Gov. Tim Walz of Minnesota said in a statement that he has been briefed on a shooting at Annunciation Catholic School in Minneapolis. “I’m praying for our kids and teachers whose first week of school was marred by this horrific act of violence,” he said.