Two children, aged eight and 10, have died after a shooter opened fire as they prayed during morning Mass at their school.
The attack, in which 17 others were injured, happened at 8:30am local time at the Annunciation Catholic School in Minneapolis, Minnesota, on Wednesday.
The children were reportedly shot at through the chapel windows as they sat praying during Mass to mark the start of the new school year. Three adults were among the injured.
The suspect has been named by police as Robin Westman, 23.
Pope Leo has expressed his "profound sorrow" over the "terrible tragedy".
In a telegram sent to Archbishop Bernard Hebda of Minneapolis, the Pope expressed "his heartfelt condolences and the assurance of spiritual closeness to all those affected by this terrible tragedy, especially the families now grieving the loss of a child".
It added that the Pope was praying for the wounded, first responders, medical personnel and clergy caring for all those affected.
The new school term had just got underway with the theme of “A Future Filled with Hope”, inspired by Jeremiah 29:11.
US President Donald Trump called the attack "senseless" and has ordered all American flags on public buildings to be flown at half-staff until sunset on Sunday in honour of the victims.
Speaking at a press conference on Wednesday, Minneapolis Police Chief Brian O’Hara called the attack “an unthinkable tragedy”.
He said the attacker "ultimately took his own life in the rear of the church".
“The sheer cruelty and cowardice of firing into a church full of children is absolutely incomprehensible,” he said.
He continued, “Our hearts are broken for the families who have lost their children, for these young lives that are now fighting to recover, and for our entire community that has been so deeply traumatized by this senseless attack.”
