Alberta's police watchdog says officers were entirely justified in their decision to shoot an armed man in a wheelchair who was firing rounds indiscriminately out of his northwest Calgary home last year.

"The man could not be allowed to continue firing shots from the residence," said Susan Hughson, executive director of the Alberta Serious Incident Response Team (ASIRT), which released the results of its investigation Thursday.

"Every shot that man fired and every moment that passed increased the potential for serious harm or death for people in the vicinity."

Police were initially called to David McQueen's home in the community of Huntington Hills around 4:30 p.m. on Jan. 24, 2016, after he fired a round from the residence that went through the driver's side window of a Calgary Transit bus, narrowly missing the driver.

The 53-year-old quadriplegic, who had limited use of his hands, went on to fire more than 30 rounds from inside the home, some of which struck nearby homes, fences, and a trailer.

Hughson said there was an "extremely high" chance of a police officer or civilian being shot.

Police had surrounded the home and, after a roughly 90-minute standoff, decided they would bombard the residence with tear gas the next time shots were fired from inside.

When that happened, tear gas canisters were fired into two doors, and Hughson said police observed McQueen exiting the rear door in his wheelchair and wielding a revolver.

"It was in his lap, in his hand, cocked and ready to fire," she said of the weapon.

Police open fire

Several officers opened fire on McQueen and it was the last round fired — a sniper's bullet — that struck him in the head and killed him, Hughson said.

She described the entire series of events as "heartbreaking," and "one of the more harrowing critical incidents" that ASIRT has investigated.

"There's no doubt that this man was in crisis on this date," Hughson said.

She said McQueen had been "struggling physically, emotionally and financially" in the days leading up to the shooting, particularly after the death of his beloved dog the week before.

Based on a phone call he made to a loved one and documents found inside the home, Hughson said McQueen appeared prepared to die in a shootout with police.

Same officer involved in another fatal shooting

One of the officers who shot at McQueen (but missed) was also involved in another fatal shooting the year before.

That officer shot 27-year-old Anthony Heffernan four times — including three shots to the head and neck — in a northeast Calgary motel room on March 16, 2015, after police received reports that a man was behaving strangely in the room.

Police said they found Heffernan high on drugs, agitated and in "medical distress" and said they were confronted with a "high-risk situation" once they broke into the motel room, which had been locked.

Once inside, officers found Heffernan standing near the beds with a lighter and a syringe in his hands. He did not obey commands to drop the items before moving toward the officers, according to the ASIRT investigation into the incident.

Heffernan's family, who are suing police over his death, have said the syringe was a "small diabetic needle" that had no tip on it and posed little threat to the officers.

ASIRT decided a year ago that the officer would not be charged criminally.

The investigation found some evidence that an offence had been committed but the Crown believed there was not enough evidence to secure a conviction against the officer.

Hughson said Thursday the officer's involvement in the Heffernan case does not impact his right to use his firearm defensively in the McQueen case.

"You have to look at the incidents independently and look at the circumstances surrounding them to determine whether the steps taken or the actions taken were justified," she said.

"And, just because the officer has been involved in another officer-involved shooting, he does not lose the protection of the law."

Calgary police issue statement

Later Thursday, Calgary police issued a statement expressing sympathy for McQueen and his family.

"The grief being endured by the family is not lost on the members of the Calgary Police Service," the statement reads. "In our duty to protect public safety, we always strive for a resolution that preserves life."

The statement went on to note that officers respond to thousands of calls each year involving people with mental health issues.

"The overwhelming majority of these calls end with us being able to de-escalate the situation and engage the appropriate partner agencies to assist in getting help to our community's most vulnerable," police said.

"This call fell outside the realm of de-escalation. Facing high-risk dynamics, all steps were taken to contain the situation, eventually leading to few tactical alternatives."