A 13-year-old boy in Glendale, Utah, was shot several times by police officers after his mother called 911 for help with his mental health crisis.
Linden Cameron, who has Asperger's, a form of autism, is now in a serious condition in hospital, his mother said.
Golda Barton said she had believed police attending on Friday night would use "the most minimal force possible".
Salt Lake City Police Sgt Keith Horrocks told reporters that the incident was now being investigated.
Speaking to local CBS-affiliate KUTV, Ms Barton said she told the 911 operator that her son needed to be taken to hospital for treatment.
He was experiencing a crisis because it was her first day back at work in almost a year and "he has bad separation anxiety", she said.
"I said, he's unarmed, he doesn't have anything, he just gets mad and he starts yelling and screaming," Ms Barton said. "He's a kid, he's trying to get attention, he doesn't know how to regulate."
At a press conference, Sgt Horrocks said officers were called to a "violent psych issue" and reports that a boy - who they did not name - had made "threats to some folks with a weapon". He added that there was no indication when they attended that the boy was armed.
An officer shot the boy when he tried to flee on foot, Sgt Horrocks said.
According to an online fundraiser set up to raise money for medical bills, Linden Cameron has suffered "injuries to his shoulder, both ankles, intestines and bladder".
"The long-term effects of his injuries are still unknown, but it is likely that his recovery will be long and require multiple kinds of treatment," the page, set up by a friend of the family, says.
According to data compiled and regularly updated by the Washington Post, 1,254 people with a mental illness have been shot dead by US police since the beginning of 2015. This represents 22% of all people shot and killed by police across the country over that period.
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STOP RUNNING FROM THE COPS ARSEHOLES.
