This story is part of Deadly Force, a CBC News investigation into police-involved fatalities in Canada.

When Hamilton Police got the dispatch call that evening in September 2016, they were told Tony Divers had just assaulted a woman, and that he was high on drugs, had a violent history and was considered "anti-police."

They were also told that the 30-year-old was armed.

When the police arrived on the scene, Divers ignored their instructions to get on the ground. Officers decided he was too close for a taser to be effective. They fired two shots, and one proved to be fatal. It turned out they had one thing wrong — Divers had not been armed.

Divers is one of 21 unarmed people who were shot and killed during encounters with police since 2000, according to a CBC News analysis.

Alok Mukherjee, a criminologist and former chair of the Toronto Police Services Board, said 21 deaths is too high.

"I don't think that any killing of a person who is in distress, not engaged in conventional violent crime, can be justified," he said.

He admitted that there may be "exceptionally rare circumstances in which deadly force may become necessary," but said, "our goal would be zero harm."

Working with limited info

The CBC data was gathered from inquests, investigation unit reports, media reports and other public records and shows that between 2000 and 2017, more than 460 Canadians have been killed in police encounters. Of those who were killed and shot, 6.4 per cent were unarmed.

(CBC)

In most of the cases captured in CBC's database, weapons were indeed involved. Knives, axes and other cutting implements show up in 33 per cent of cases of armed interactions with police. Nineteen per cent of those killed had a firearm on them at the time, while six per cent had a replica firearm.

Typically, police receive limited information from dispatch operators before proceeding to a possible crime scene. The raw details usually come from 911 calls from witnesses whose information may not be reliable.

In some cases in the database, people were at first deemed to be "armed," but the weapons ended up not being what they at first appeared.

In 2015, Toronto Police believed Rodrigo Hector Almonacid Gonzalez, who had barricaded himself in a bathroom, was armed with a screwdriver. After a SWAT team stormed into the room and shot him, they discovered he had only been armed with a toilet seat cover.

In 2005, Ontario Provincial Police officers responded to a call that 20-year-old Jason Steacy had stabbed someone. When they confronted him in his trailer, he was sitting in front of his computer. Believing he had a gun in his hand, they shot him. It turned out to be a computer mouse.

Tragic misunderstandings

Such misunderstandings aren't just tragic for the families of the victims. They also deeply affect the people who pull the trigger, such as Syd Gravel, now a retired Ottawa police officer.

"I remember falling back up against the car and kind of feeling like I wasn't trained to shoot people that weren't a danger, and I couldn't have known any better at the time," said Gravel. Now a retired Ottawa police officer, Syd Gravel developed PTSD after shooting and killing an unarmed suspect in 1987. (CBC)

In 1987, Gravel and his partner responded to a robbery call in an Ottawa neighbourhood. It was 3:30 in the morning, but there were people around. In addition to the suspects, there was a man walking his dog and a woman sitting on her porch.

Gravel demanded the suspects show their hands so he could see whether they were armed or not. When one of the suspects didn't immediately comply, Gravel said his training kicked in.

"He started turning towards both my partner and the lady on the porch," Gravel said. "And I couldn't allow him to complete the turn not knowing what was in his hands, so my decision at that point in time, for the safety of others and myself, was to shoot."

Gravel was devastated when he discovered the man was unarmed. The incident haunted Gravel, who developed PTSD.

"Every moment that I put that gun on [after that], I often wondered each and every time if this would be another day that I would have to use this thing, and it would cause great trepidation in whether or not I wanted to continue to work," Gravel said.

What does 'armed' mean?

Sometimes the concept of "armed" is complicated.

Roy Wellington's 15-year-old nephew Duane Christian was shot and killed by police in June of 2006 while he was behind the wheel of a stolen vehicle. Although Christian did not have a weapon, Toronto Police believed he was driving the car in a threatening manner. Fifteen-year-old Duane Christian was shot and killed by Toronto police in June of 2006 while he was behind the wheel of a stolen vehicle. (Christian family)

"I personally don't think my nephew was trying to hurt anybody. I don't think he was going to use that vehicle as a weapon at all," Wellington said. "I think he was doing what a scared 15-year-old person might do if someone pointed a gun in their face and a flashlight, and that is run."

Wellington believes police failed his nephew.

"These officers forgot my nephew is a member of the community and placed the importance of recovering a stolen vehicle over the value of a life. To me that's pretty callous," he said.

"I don't believe that these officers have a real commitment in these moments of making sure that everyone survives this encounter. I believe that that is a failing of policing."

'A constant reassessment'

RCMP were involved in 118 of the fatalities in the CBC database, and 14 per cent of the people shot and killed by RCMP were unarmed — more than twice the national average. Recently, they have made an effort to better train officers.

Police say that in a potentially hostile encounter, things can change fast and perception can be altered by external factors, which is one of the things the RCMP teaches its cadets at its Regina Training Academy.

"Things change second to second to second. It's a constant reassessment, it doesn't stop," said Cpl. Rob King, one of the instructors. "A police officer responding has 50, 60 different operational factors they have to take into consideration before they can determine what may happen, where they can go next and what the proper intervention option may be to try and get that situation under control."

Cpl. Rob King helps train RCMP officers in de-escalating high-stress situations. (CBC)

Fellow instructor Cpl. Rob Bell said situational factors can include lighting conditions, being outdoors versus indoors and the distance between you and a suspect.

Cpl. Bell said among the questions that an officer needs to address is, "What do I want to do here? What equipment do I need? Is the taser or conducted energy weapon available to me? Is the patrol car van required? Do I need the police dog services or emergency response team?"

Cadets are taught de-escalation tactics that start with communication.

"You're always talking. Never stop talking," Cpl. King said.

They are also put through simulator training that teaches them how and when to apply force. Cadets are faced with different scenarios on an interactive screen and armed with an electronic pepper spray and pistol.

As the scene unfolds, they have to decide when it is appropriate to apply force. There is a moment in which the window to use non-lethal means closes, and lethal force becomes the only option.

"The option of using lethal force is a last resort and can only happen in very specific circumstances. There has to be that threat of grievous bodily harm or death," Cpl. Bell said.

'Break that tunnel vision'

The RCMP is also training officers how to de-escalate themselves by coaching them in combat breathing techniques.

"It's a way to break that tunnel vision. It's taking in a deep breath and holding it, releasing it and pausing before you take the next one," Cpl. Bell said. "It brings the heart rate down, it breaks the tunnel vision and it allows [officers] to regain their focus and make sure that they are getting a complete picture."

CBC's data analysis shows fatal encounters with police are on the rise, and that there are few consequences for officers who use excessive force. Of the 461 deaths, there were 18 instances of officers being criminally charged, and only two convictions.

That troubles Syd Gravel, who was cleared of any wrongdoing following his fatal encounter with an unarmed man.

"You have to be able to justify it," Gravel said. "At the end of the day, you're going to be held responsible for how you behaved, reacted and the end result."

​​Got a tip for the CBC News I-Team? Email iteam@cbc.ca or call the confidential tip line at 204-788-3744.