Police have laid several charges against a man blamed for causing a bus accident that injured all 32 passengers – but the police are also being investigated.

Ontario's Special Investigations Unit, which investigates police, is reviewing what role officers may have played in the crash on Dec. 23.

A man grabbed the steering wheel of a Greyhound bus, causing it to swerve off the road and land on its side in a ditch near Thunder Bay, Ont.

The SIU wants to find out if police put the suspect on the bus or if he got on it voluntarily. The unit will decide if a full-blown investigation is needed after reviewing the case this week.

Louise Lent, 74, remains in critical condition in hospital with injuries that include a punctured lung and broken ribs.

Five other seriously injured passengers have been treated and released.

Shaun Davis, 22, has been charged with mischief and criminal negligence causing bodily harm.

Davis was on another bus when he demanded to get off, and called police. He believed that he was being followed.

After police responded to his call, they escorted him to the Greyhound bus at Ignace, Ont., a town northwest of Thunder Bay.

"The police, I sat and I listened to them tell the two younger boys who were to sit behind this fellow not to look or talk to this individual in order not to agitate him," recalled Thayne Gilliatt, who was on the bus that night.

Passengers say the police assured everyone on board that Davis was not violent. But Pam Meady didn't believe it.

"I was afraid with the way he was acting he would make a lunge for one of us," she said. "I didn't want to be a target."

Meady and Gilliatt say Davis was increasingly agitated, and stood in the stairs at the front of the bus for a while before the crash.

The bus was travelling from Winnipeg to Toronto.