TORONTO -- The family of a 26-year-old Brampton, Ont. man is calling for more mental health training for police officers, after he was stunned with a conducted energy weapon and shot dead by police.

The victim, who has been identified by family as D'Andre Campbell, was killed inside his home on Sawston Circle, in the area of Edenbrook Hill Drive and Bovaird Drive, after Peel Regional Police were called to the location around 5:30 p.m. on Monday.

The Special Investigations Unit (SIU), which has taken over the probe, said preliminary information indicates that two officers discharged conducted energy weapons before one of the officers fired his gun multiple times.

Campbell's eldest sister, Michelle Campbell, said she is traumatized after witnessing the incident.

"He was Tased twice. He was already on the floor," Michelle told CTV News Toronto on Tuesday. She said she was in the basement when she heard commotion and went upstairs to find two police officers in her home.

"I was standing right there," Michelle said. "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 said her brother was holding a knife at the time but did not move towards the officers.

According to the family, D'Andre Campbell was the one who called police to the home Monday. They said he had mental health issues and that police have been to the home numerous times in the past, so they should have known they were dealing with a mental health patient.

“That’s why I’m saying I’m confused," Dajour Campbell, D'Andre Campbell's brother, said. "They came to the house multiple times. I don’t know why this time they decided to shoot him.”

"Every crisis scenario is different," Peel Regional Police Constable Akhil Mooken said in an interview with CTV News Toronto.

“One of the challenges we have as front line officers, is that because of how dynamic an incident can be or how quickly we arrive on scene, we don’t necessarily know all available information before entering into that situation. Every call potentially has its unique challenges.”

Peel Regional Police said they attend more mental health calls than any other type of call, and that they have nearly 20 partnerships, training courses and initiatives to support officers on the front lines and to help individuals in crisis.

"Our officers are constantly getting training on different mental illnesses, and what the best way is for our officers to interact with individuals who are in crisis," Constable Mooken said.

But the Campbell family says they want justice. They are calling for the police service to review their mental health initiatives, and improve training for officers.

D'Andre was the third of eight siblings, who all lived in the Brampton home with their parents.

"He's a fun-loving guy," Campbell's aunt, who did not want to be identified, told CTV News Toronto. "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."

The SIU has designated one subject officer and four witness officers in the case, and has assigned four investigators and two forensic investigators. The SIU is called in whenever police are involved in a death, serious injury or allegations of sexual assault.

The post-mortem is scheduled for April 8.

Anyone with information about the case is being urged to contact the lead investigator at 1-800-787-8529.