The sister of a young man with mental health issues who was stunned and then fatally shot by police at his home in Brampton Monday evening says the incident inexplicably escalated “within seconds.”

“I was literally, I was standing right there. I turned this way and I turned back, and by the time I turned back the officer had the gun in his hand, and within seconds, he shot him,” Michelle Campbell told CTV News in an interview Tuesday.

She said her brother “didn't move” before being stunned to the floor.

“He didn't attack anybody. He didn't say anything. He had a weapon in his hand, a knife in his hand, but he stood there.”

D’Andre Campbell, 26, was pronounced dead Monday night.

He had been home with his mother and several siblings when police responded to their house on Sawston Circle shortly after 5:30 p.m. after receiving a call about a domestic disturbance – a call D’Andre’s family said he placed himself.

According to the province’s Special Investigations Unit, an arm’s length agency that investigates any case where police are involved in a death or serious injury, two officers stunned D’Andre with conducted energy weapons before one of the officers fired multiple shots at him.

Michelle Campbell, D’Andre’s eldest sister, said she was in the basement when she heard a commotion and came upstairs.

She said she saw her brother get stunned twice before he was shot.

"He was Tased twice. He was already on the floor," she said.

Crews on scene attempted CPR, but couldn’t save the young man.

D’Andre’s brother, Dajour Campbell, spoke with CP24 at the scene Monday night and said that police had been called to the home multiple times in the past due to his brother’s mental health issues.

“That’s why I’m saying I’m confused. They came to the house 1,000 times, multiple times. I don’t know why this time they decided to shoot him,” he said.

Michelle Campbell said the family is now looking for answers and wants to see better training for police on how to deal with mentally ill people who are in crisis.

Peel police said Tuesday that they do have a number of programs designed to teach officers how to deal with people in crisis.

“We here in Peel Region have nearly 20 partnerships, training courses, and different initiatives to support our officers on the front line and to help individuals who are in crisis get the support that they need,” Const. Akhil Mooken said.

As per protocol, the police force will not comment on a specific case while the SIU is investigating.

But Michelle Campbell said the training that currently exists may not be enough.

“They need to give right training. They had two young officers,” she said.

Campbell's aunt, who did not want to be identified, called him "a fun-loving guy."

"He's not an aggressor, but with someone with a mental illness, things do happen because they're not able to control their emotions at times," she told CTV News Toronto.

Vernol Gordon, a family friend, said D’Andre was the sort of guy who “loved to socialize.”

He added that the tragic situation is a ‘no-win’ situation for everyone.

“I feel sorry for the officer. I feel really bad for the family. There's no win-win in this one,” Gordon said.

D’Andre was the third of eight siblings.

Four investigators and two forensic investigators have been assigned by the SIU to probe his death. The SIU has designated one subject officer and four witness officers in the case.

- With files from CTV News Toronto’s Tracy Tong