Ontario's Special Investigations Unit has again reopened its probe into the arrest of a man whose arm was broken during the G20 summit in Toronto last summer.

The investigation has twice been dropped – on November 25 and again on May 16 – after investigators did not receive the names of the officer or officers involved in the incident.

Dorion Barton, 29, claims that his arm was broken when he was assaulted by police during last June's G20 demonstrations in downtown Toronto.

Civilian witnesses told the SIU that Barton was taking pictures of mounted police officers when he was struck with a riot shield and hit with a baton or batons while lying on the ground.

Eleven police officers who had been identified at the scene were also interviewed by the SIU, but none could identify the perpetrator or perpetrators of the strikes against Barton. The arms-length agency stopped the investigation last week.

On Thursday, SIU director Ian Scott said the investigation had been reopened after Toronto Police Chief Blair promised to provide the name of a person who was able to identify the subject officer – four months after the SIU had requested it.

"In anticipation of the timely receipt of this new information, the Unit is reopening the Barton investigation," Scott said in a release.

Toronto lawyer Clayton Ruby says Blair had been uncooperative with the probe into injuries his client allegedly sustained while being arrested by riot police on June 26.

Clayton told CTV Toronto's Canada AM on Thursday that Blair did not seem to enforce "command and control" over his force.

Ruby said it was suspicious that 11 officers who had been on hand for the incident were unable to provide any help when interviewed.

"It appears all 11 officers who were interviewed were struck with a sudden case of blindness," Ruby said. "We call it ‘thin blue line disease.' It means you don't see anything that is going to harm another officer. It is caused by a police chief who doesn't have sufficient control over his force."

When asked for comment, Toronto police spokesman Mark Pugash said Ruby was simply trying to smear the chief's name.

"Mr. Ruby is looking for cheap targets. He enjoys indulging in cheap insults and I think we will treat it as that," Pugash told CTV Toronto.

The SIU is an arm's-length agency that investigates reports involving police where there has been death, serious injury or allegations of sexual assault.

With files from The Canadian Press