Police officers are seen arresting Adam Nobody, 27, during a G20 protest at Queen's Park in Toronto on June 26. (YouTube)

Ontario's police oversight agency is reopening an investigation into alleged police misconduct in the case of a man who suffered facial fractures while being arrested at a G20 protest in Toronto.

The Special Investigations Unit said in a Tuesday release it will take a fresh look at the case of Adam Nobody, 27, whose nose and cheekbone were broken during a June 26 protest at Queen's Park.

The SIU's initial investigation determined that excessive force was "probably" used, but it couldn't determine which specific officer was responsible for causing the injuries.

In arriving at that conclusion, the SIU interviewed a civilian witness and eight officer witnesses, as well as reviewing a video of the arrest that was posted on YouTube.

The video shows about a half-dozen police officers chasing and then tackling Nobody at Queen's Park. SIU director Ian Scott said the video appeared to show one of the officers striking Nobody repeatedly while he was on the ground.

Toronto police Chief Bill Blair criticized the SIU for using a video that he said had been edited and "tampered with."

"The evidence that they're relying on is false," Blair told CBC Radio's Metro Morning on Monday.

The man who filmed the YouTube video, John Bridge, sent a sworn affidavit to the SIU saying the video was not edited. He said he had just turned off his camera for a few seconds as he considered backing away from advancing police officers.

"In my view, the assertions by Chief Blair and the contents of the affidavit sworn by Mr. Bridge satisfy the criteria of being materially new information with respect to the Nobody incident," said Scott in a statement. "Accordingly, I am reopening this investigation."

Lawyer asks for police evidence

The SIU, which investigates incidents in which civilians are killed or seriously hurt in interactions with police in Ontario, will also interview Bridge. Investigators will also ask Blair to turn over "any further relevant information he has with respect to this incident and more specifically any forensic evidence in his possession regarding the allegation of tampering with the video tape."

Nobody's lawyer Julian Falconer dismissed Blair's criticisms.

"I sure hope the chief of police, having made these very serious allegations has something to back them up," he said Tuesday.

"You hope this wasn't the rant of an overly defensive police chief."

The SIU also investigated five other cases where people alleged mistreatment at the hands of police during the G20. In one of those other cases, the SIU found officers had likely used excessive force.

But the watchdog is not proceeding with criminal investigations on any of the complaints, citing a lack of evidence or an inability to determine how exactly the complainants sustained their injuries.