Hilary Bonnell disappeared from her northern New Brunswick community in September 2009. (RCMP)

A respirologist says it's plausible Hilary Bonnell died from someone sitting on her chest and placing their hand over her nose and mouth.

Dr. Graham Bishop, who testified on Tuesday during the first-degree murder trial of Curtis Bonnell, said it was his opinion that Hilary could have died the way police believe.

Under cross-examination, however, Bishop said it was also possible the 16-year-old could have somehow self-smothered under the effects of alcohol and drugs.

Without "definitive proof," such as hand prints or video surveillance, it's impossible to say exactly how she died, Bishop said.

Bonnell, 32, of the Esgenoopetitj First Nation, is charged in the death of Hilary, his first cousin.

The Crown alleges Bonnell picked up Hilary on Sept. 5, 2009, as she was walking along Micmac Road in the province's northeastern community after a party.

Bonnell is accused of holding Hilary against her will, sexually assaulting her and killing her.

Did not examine body

Bishop told the jury he had been asked by the lead investigator RCMP Cpl. Greg Lupson to give an opinion on Hilary's death.

Lupson had suggested that someone sat on Hilary's chest and placed their hand over her nose and mouth, said Bishop.

Bishop said that would be consistent with smothering as the cause of death.

He did not actually examine Hilary's body, he said. He relied on the autopsy report.

The pathologist who conducted the autopsy was unable to determine Hilary's exact cause of death, but concluded the probable cause of death was asphyxia.

Earlier this month, the jury watched a police video, where Bonnell admitted he killed Hilary in his backyard after they had sex.

Bonnell told police they fought because she wanted $100 in exchange for the sex and he refused. He said he covered her mouth to stop her from yelling and the next thing he knew, she was dead.

When someone is smothered, the first three to five minutes is the panic stage, the respirologist told the Miramichi courtroom.

The person usually falls unconscious within five to seven minutes, followed by cardiac arrest.

Beyond seven minutes causes brain damage and death. If no one intervenes at that point, it's "hopeless," said Bishop.

Alcohol or drug use would make the situation worse, he added.

The trial started on Sept. 17 and is scheduled to last up to eight weeks.