Christopher Garnier put his hands on Catherine Campbell's throat and heard her last gasps before she died, a jury heard Thursday as they watched a video recording of his 2015 police interrogation at his second-degree murder trial in Halifax.

Det. Const. Michelle Dooks-Fahie asked about the reasons Garnier heard "two gasps" before Campbell died. 

"My hands were on her neck," Garnier responded, adding he didn't know for how long.

The Crown alleges Garnier strangled Campbell — an off-duty police officer with the Truro Police Service — in an apartment on McCully Street in central Halifax, then transported her body in a green compost bin to a steep embankment near the Macdonald Bridge.

Garnier is on trial in Nova Scotia Supreme Court in Halifax for second-degree murder and interfering with human remains. He has pleaded not guilty to both counts.

Catherine Campbell

Catherine Campbell, an off-duty police officer, was strangled and her body dumped in a green bin on Sept. 11, 2015. (CBC)

RCMP Cpl. Jody Allison, one of the officers who interrogated Garnier, testified Thursday that he didn't know the cause of Campbell's death when he started the interview. All he knew, he said, was that an autopsy was underway.

The defence has suggested her death was accidental and the result of a consensual sexual encounter involving choking.

On Thursday, the jury finished watching the 9½-hour video of Garnier's interrogation following his arrest on Sept. 16, 2015, hours after Campbell's body was found.

'She wasn't moving': Christopher Garnier interrogation video12:11

Toward the end of the video, Garnier said he punched Campbell possibly three times and might have already have had a hand on her throat at that point. When asked by investigators why he did it, Garnier said he didn't know.

Police also asked if Campbell struggled but Garnier said he didn't remember, although he wondered if a possible struggle might explain why his neck chain was missing. As the interrogation went on, police told Garnier he couldn't have "blackouts of convenience."

Garnier wrote to Campbell's family

About 8½ hours into the interview, police left Garnier in the room alone with a pen and piece of paper. He took a long drink of water, hugged his arms into his stomach and gazed toward the floor before picking up the pen.

Garnier, who later told the officers the letter was addressed to Campbell's family, wrote, "I never wanted this to happen" and "I don't expect you to forgive me for what happened, so I won't ask for forgiveness."

Another part of it read: "If I could give my life to get hers back I would."

Christopher Garnier note

The note Garnier wrote during his interrogation. (Court exhibit)

Earlier in the interrogation, Garnier told police he remembered Campbell lying on a mattress in the apartment, her face covered in blood. He repeatedly said he did not know why her face was bloody.

He told police he heard her take her last breaths, but struggled to remember details of the night she died.

"I don't know how this happened," he said at one point. "I've been trying to remember what happened."

Garnier said he didn't remember how Campbell got on a mattress or what became of it afterward, but he did remember there was blood all over the back of it. Police never found the mattress.

Garnier told police that Campbell was dead when she was placed in the compost bin. When asked how he knew, Garnier said she wasn't moving or breathing.

The trial will resume next week. 

Blair Rhodes live blogged from court. Mobile users can view the blog here.

with files from the Canadian Press