Const. Babak Andalib-Goortani arrives at the Toronto office of the SIU on Wednesday morning. (Toronto Star/thestar.com)

Ontario's Special Investigations Unit says it knows the names of all three police officers involved in an alleged assault during last summer's G20 summit — but without independent witnesses it can only lay charges against one officer.

Const. Babak Andalib-Goortani, attended the SIU office on Wednesday morning accompanied by legal counsel, and was served with a summons and had the charge of assault with a weapon explained to him.

The charge is related to the arrest of Adam Nobody.

The 27-year-old was at Queen's Park on the evening of June 26 when he was caught up in one of the anti-G20 protests.

A YouTube video of the incident shows Nobody being chased across one of the lawns by a group of police. He is taken to the ground and the video shows at least one police officer making a punching motion.

Adam Nobody suffered a broken nose and a facial fracture during his arrest.

When he was released from detention, Nobody pursued two charges of police brutality against the arresting officers.

The SIU, an arm's-length agency that is called in whenever there are situations of death or serious injury involving police, conducted the investigation.

But in November the SIU said, although it believed excessive force had been used against Nobody, it could not identify the suspect officers.

Adam Nobody suffered a broken nose and a facial fracture during his arrest in June. (CBC)

The SIU later received other videos which it said helped identify one officer.

On Tuesday, the SIU announced they had a witness — and enough evidence — to proceed with a charge against Andalib-Goortani.

But SIU investigators said they also have evidence against two more officers. The trouble is they can't identify them and despite interviewing 12 other officers — described as witness officers — their identities remain unknown.

"Well, one officer can be seen striking quickly, two or three times, towards Mr. Nobody in the area of his upper body or head," said Frank Phillips, spokesman for the SIU.

"Their actions — in the view of the director — they could have caused the serious injury and it was excessive."

For now, the two suspect officers will not face criminal charges but Phillps confirmed to CBC News the SIU knows the names of the pair and says they will face charges if the SIU can find independent witnesses able to identify them.

"We viewed a number of videos, still photography. I'm sure there's a number of people out there that have video that's never surfaced. If we got more information that helped narrow that down, then [SIU director Ian Scott] may choose to reopen the investigation and we'd go from there," said Phillips.

Nobody's lawyer, Sunil Matthai, said Toronto police Chief Bill Blair needs to step in and break the wall of silence.

"Somehow [Toronto police] were able to find the identity the 90 officers who were not wearing their [name] badges that day but when it comes to identifying three individuals who were involved with Mr. Nobody's case, somehow they are unable to do so."

Key details may be contained in the notebooks of the officers who were in the area where Nobody was allegedly assaulted on June 26. But the notes are beyond the reach of the SIU.

Blair said those notes will be a matter for an internal police investigation.