The Special Investigations Unit shouldn't lack for witnesses of what happened Sunday.  Dozens of people have come forward with snippets of what they saw and heard.

But it will be up to the SIU to figure out whether those interpretations are indeed accurate. The SIU, in its months-long investigation, will try to figure out what role the police played in the death of Abdirahman Abdi; whether their tactics were excessive or whether they followed their specific mandate on the use of force.  Ask many of those who saw it that day and they'd say no.

His first name is Rey and that's all he'll give us along with his version of what happened Sunday morning.

“I heard the screaming, cops yelling stay down, stay down," Rey tells us, "The guy's down man, and they're just boom boom boom boom boom, hitting him.”

The aftermath of that encounter was captured on camera by witnesses, many of whom believe police overreacted.

"They hit him, they beat him," says Zeinab Abdalhallah, "until he was dead."

Carleton University criminology professor Ron Melchers, who has studied the use of force among police officers, says in past cases, witnesses statements aren't often verified.

“It's very difficult for bystanders to put themselves in the position of a police officer. You can't armchair quarterback these kinds of calls,” he says.

The head of the police association Matt Skof says this is a tragic situation, "forced was used, and that's always going to be questioned," he says.  But speculation about racism, Skof says, is not appropriate.

“Unfortunately what we're seeing is this bleed of rhetoric from the U.S. and it's not an appropriate conversation; we don’t have the same history, not same relationship with public.”

In the meantime, groups like the National Council of Canadian Muslims is urging people to wait for the results of the investigation before drawing conclusions.

“No one can determine what was at play until we've got all facts once the investigation is complete” says Amira Elghawaby from the National Council of Canadian Muslims. “It's not the time to speculate.”

But Rey says he knows what he saw and he's sticking to it.

“If I had been 30 years younger, I would have jumped that cop that's how angry I was. It wasn't called for; the man was dow.  You don't beat up on a man when he's down.”

The SIU confirmed today that the autopsy has started on Abdi's body but other tests, including toxicology tests, will take much longer.  Ottawa Police also confirmed that the seven police officers involved in this incident are all still on active duty.