Children become emotional as they listen to speakers during a candlelight vigil at a park for victims of the shooting at the Annunciation Catholic School in Minneapolis, Minnesota, on Wednesday.

MINNEAPOLIS — An assailant armed with three guns fired through stained-glass windows into a Catholic church where parish school students were attending Mass on Wednesday, killing two children and wounding 17 other people, officials said.

The shooting ended when the lone suspect, identified as Robin Westman, 23, "took his own life" at the rear of the church, according to Minneapolis Police Chief Brian O'Hara, who declined to offer a possible motive for the attack.

A videotaped message by the suspect showed Westman struggled with depression and was fascinated by the perpetrators of past mass shootings.

FBI Director Kash Patel said his agency was investigating the attack as an "act of domestic terrorism and hate crime targeting Catholics".

Two victims, aged 8 and 10, were slain where they sat as the gunfire turned the morning Mass into pandemonium. It sent worshippers diving behind pews for cover while older children scrambled to shield younger ones, officials said. At least two of the church exits were blocked by wooden planks barricaded outside the doors, O'Hara said.

The violence struck at the start of an all-school Mass held annually on the first Wednesday of the academic year at Annunciation Catholic School.

"This was a deliberate act of violence against innocent children and other people worshipping. The sheer cruelty and cowardice of firing into a church full of children is absolutely incomprehensible," O'Hara said.

In addition to the two children killed, 17 other people were struck by gunfire — 14 of them students aged 6 to 18 and three parishioners in their 80s, O'Hara said.

A 2017 yearbook from the school showed that Westman, who went by the first name Robert at the time, had been a student there, the Minnesota Star Tribune reported.

Online videos reviewed by Reuters showed the text of a suicide note in which the shooter expressed feeling depressed and wanting to carry out a mass shooting. Names of previous school shooters were scrawled on a rifle magazine, along with erratic and wide-ranging political grievances.

In a statement on X, US Homeland Security Secretary Kristi Noem said the suspect was "claiming to be transgender".

Appearing with the police chief and other officials at a news conference on Wednesday afternoon, Minneapolis Mayor Jacob Frey cautioned against bringing gender politics into the tragedy. Frey also cited the easy availability of firearms as a root cause of mass shootings that are commonplace in the United States.

The shooting at Annunciation, a parochial school with about 395 students, marked the 146th incident of gun violence at a place of primary or secondary education since January, according to the K-12 School Shooting Database.