Canada's federal prison watchdog, which has been investigating the 2015 death of a Cape Breton man while he was in custody last year, is concerned about the "rapid escalation of force" used against Matthew Hines.

Howard Sapers says the details of Hines's death — which were secret until reported by CBC News this week — caught his attention "immediately" after the incident on May 27, 2015.

Sapers, the correctional investigator of Canada, has been investigating how and why guards beat Hines and pepper-sprayed him repeatedly before his death. A board of investigation report into 33-year-old Hines's death details the force they used and has deemed it "inappropriate."

But Sapers says his office's investigation has been held up by one missing piece of the puzzle: a report from New Brunswick's coroner's office that could list a definitive cause of Hines's death.

"It's been over a year," Sapers said.

"I don't know whether that means this will be going to a full inquest or not. But it does concern me even that the family has to wait this long for that kind of very basic information."

Paul Bradley, a spokesman with New Brunswick's Department of Public Safety, declined to comment on why 15 months after Hines's death, the coroner still hasn't filed a final report. That, he said, is private health information, even though Hines is deceased.

If Hines had been incarcerated in a federal prison in Ontario, an inquest into his death would be mandatory.

But because he died in a New Brunswick prison, the decision to call an inquest is up to the province's chief coroner.

Bradley wouldn't comment on whether the office is considering calling an inquest in the Hines case.

Hines 'likely' died from pepper spray

Hines died in May 2015 and according to a post-mortem report, the cause of death was likely lack of oxygen after being pepper sprayed five times by guards. 2:04 A post-mortem report found Hines likely died from a lack of oxygen, caused by being pepper-sprayed.

CBC News has examined a board of investigation report into Hines's death, an internal report written in most cases of deaths in federal custody.

The report describes how guards hit and pepper-sprayed Hines several times when he refused to return to his cell.

He was taken to the Moncton Hospital in medical distress, after having difficulty breathing.

Hines was pronounced dead less than two hours after the altercation with guards began.

A news release issued after Hines's death says he was "found" needing medical attention and staff "immediately" delivered CPR.

But the board of investigation report tells a different story.

He wasn't "found" needing medical attention. The report says he was with guards when he started having medical distress.

Hines was moved to the health wing of the prison, but the report says the nurse on duty didn't give him "the required medical assessment and treatment."

The report also says CPR was performed when he was in an ambulance, headed to the hospital.

Correctional Service Canada hasn't explained why the original story differs from the one in the report, nor has anyone from the correctional service agreed to do an interview about Hines's death.

CSC spokeswoman Lori Halfper said officials can't discuss specifics of Hines's death due to privacy legislation.

She said changes have been made as a result of the death, but did not disclose what those changes were.

A spokesman for Public Safety Minister Ralph Goodale declined to comment on Hines's death, as it's still under investigation by CSC.

Circumstances caught watchdog's attention

Howard Sapers, Canada's correctional investigator, is investigating the death of Hines. (CBC) Even if an inquest isn't called, Sapers has the power to dig deeper into a case of death or serious injury of a prison inmate.

His office has been investigating Hines's death for more than a year.

As part of his investigation, Sapers has watched videos of the struggle between Hines and correctional staff.

CSC has declined to provide copies of those videos to CBC News, saying it could jeopardize its investigation.

Sapers says he has concerns about the "physical handling" of Hines and the "lack of medical followup once there were obvious injuries."

At one point, Hines was pepper-sprayed in the face four times by the same guard, with the bursts only seconds apart.

To be sprayed in quick succession like that is "unusual," Sapers says.

It's part of broader concerns he has about use of force inside federal institutions, particularly when it comes to pepper spray.

"For me, [Hines's death is] another very, very graphic example of when security-driven responses actually interfere with health-care responses," Sapers says.

Family devastated by brother's death

Wendy Gillis and Helen MacLeod want justice for their brother, Matthew Hines. (CBC) Hines's sisters want someone to be held accountable for their brother's death. He was only a few months shy of completing a five-year sentence for robbing a bank when he died.

"They should have just walked away, gave him five minutes, and Matthew would have went to his cell," says Helen MacLeod.

In a statement, CSC said that mental health treatment is a priority for the agency, noting it spent $85 million on mental health services in 2014-15.

Hines's sisters also question why they had to wait 13 months to find out the details of their brother's death.

"I could not believe what Matthew went through. It was just horrible," Wendy Gillis says.

Sapers agrees that families shouldn't have to wait so long to learn basic details of how their loved one died.

A report he recently filed said CSC often lacks compassion and isn't transparent when dealing with families of deceased inmates. He made nine recommendations on how CSC could do better.

Sapers says he hopes the Hines family gets the answers they're looking for.

"Mr. Hines's death is a tragedy in many respects," he says.

Do you have any story ideas for the CBC New Brunswick investigative unit? Send them here