A Calgary police officer has been convicted of assault after delivering what's been described as "tremendous violence" to an unarmed suspect who ended up with a collapsed lung and broken ribs.

Clayton Prince was unarmed when he was arrested by Constables James Othen, Kevin Humfrey and Michael Sandalack in July 2016.

Othen was found guilty of assault causing bodily harm and assault with a weapon while Humfrey and Sandalack were acquitted of all charges by Judge Margaret Keelaghan, who delivered her decision Friday afternoon.

"I find that the use of force by Othen was excessive, unnecessary and clearly disproportionate to what was required in the circumstances," said Keelaghan.

Photos show some of the injuries Clayton Prince sustained when he was arrested by Calgary police officers who now face criminal charges. (Clayton Prince)

Video cuts off

The Calgary officers were each charged with assault causing bodily harm. Othen also faced charges of mischief and assault with a weapon for allegedly digging a key into Prince's neck, while Humfrey was also charged with two counts of public mischief. The mischief charges related to false statements the two officers are alleged to have given after the incident.

The Calgary Police Service began its investigation after dash-cam video from one of the responding cruisers was discovered depicting the officers delivering knee strikes and punches to Prince, who had surrendered on the ground.

The video cuts off after about eight seconds.

In her decision, Keelaghan said she found it "very concerning and troubling that the in-car video cameras in two instances were abruptly terminated at pivotal points during this incident."

Video entered as evidence at the trial of three Calgary police officers accused of assault after a traffic stop. 0:07

The arrest happened after Prince ran from police during a traffic stop because he says he was paranoid after taking cocaine and marijuana and had been driving without a licence.

Initially, Prince hid in some trees but was flushed out by more officers who arrived on scene. When one officer pulled a gun, Prince surrendered on the ground with his hands on his head, complying with police demands. That's when the video shows officers beginning to deliver punches and knee strikes.

Prince testified he was hospitalized for five days with broken ribs and a collapsed lung after the officers "beat the shit" out of him. On one of the videos played during the trial, he can be heard screaming in pain.

The video cuts off soon after Othen jumps on Prince's back and begins to punch him.

Several police officers testified for the prosecution, including CPS members who described the arrest as "out of control" and "extremely excessive."

Trouble with testimony

During the trial, Othen testified he believed the situation required a "dynamic takedown" and that Prince was resisting.

Keelaghan found that testimony was not credible and said in her decision that it contradicted other evidence, including the video and other officers' testimony. She said Prince had "complied with the commands, got on the ground and waited to be handcuffed" with his hands on his head when Othen jumped on his back and caused his lung to collapse and ribs to break.

"I do not find Const. Othen's evidence is reliable," said Keelaghan.

The judge also said she did not believe most of Humfrey's testimony and even suspected he delivered blows to Prince after he was handcuffed. She said she was left with enough doubt as to his intentions regarding reasonable force.

And although Sandalack's testimony was "lacking in credibility" and there were elements that Keelaghan said she did not believe, she also wrote that "it would be unsafe to convict on the whole of the evidence."

CPS reacts to conviction

Calgary police Chief Roger Chaffin said he can't comment on this file because the charges were laid by the Alberta Serious Incident Response Team (ASIRT), but he did release a brief written statement.

"As with a member of the public, when an officer is charged with an offence, they have the right to a fair trial. I fully respect the judicial process and will take time to review the verdict."

Othen will continue to be relieved from duty without pay while the other officers' statuses with CPS are now under review.

Under the Police Act, an internal investigation will now be conducted by the professional standards unit.