HALIFAX -- Christopher Garnier told police he put his hands on Catherine Campbell's neck and removed them after hearing gasps before the off-duty police officer died, his murder trial has heard.

Garnier told investigators during an interview hours after his arrest that he put pressure on her throat.

"My hands were on her neck," Garnier said, adding that he could not recall for how long.

The jury finished watching the 9.5-hour-long interview Thursday at Garnier's murder trial in Nova Scotia Supreme Court. It also heard that Garnier wrote an apologetic letter to Campbell's loved ones during the interrogation.

The 30-year-old was arrested by police early on Sept. 16, 2015, around the same time Campbell's body was found face down in thick brush near Halifax's Macdonald Bridge. He was interviewed by police later that day.

Garnier cried throughout much of the taped interview as he told an interrogator he saw blood coming from Campbell's nose and he "could hear her take her last breaths."

He repeatedly told police during the interview that he couldn't remember why Campbell's face was bloody.

Towards the end of the interview, Det. Const. Michelle Dooks-Fahie asked about the reasons he heard gasps before she died.

Garnier said his hands were on her neck, and that he removed them when he heard gasps.

"I don't think she was (moving) very much at that point," he said.

Garnier told the officer, "It doesn't make sense to me."

"I didn't know what I was doing until I was already doing it and it was too late," said Garnier, who repeatedly told interrogators that his memories of the night are fuzzy.

He was asked to describe how his hands were on Campbell's neck, and held his hands out in front of him, his fingers fanned out and his thumbs touching.

Garnier has pleaded not guilty to second-degree murder and interfering with a dead body.

Campbell was seen kissing and dancing with Garnier at a downtown bar before leaving with him in the early hours of Sept. 11, 2015.

The Crown alleges Garnier punched and strangled the 36-year-old Truro, N.S., police constable in an apartment on McCully Street that same day, and used a compost bin to dispose of her body near the bridge.

Last week, the defence put forth a hypothetical scenario suggesting Campbell died during a consensual sexual encounter after encouraging Garnier to choke her.

After roughly 8.5 hours of the police interrogation, Garnier was left alone in the room with a pen and piece of paper.

He can be seen taking a long drink of water, hugging his arms into his stomach and gazing towards the floor before picking up the pen.

Garnier wrote that he has "always been a caring person, but this is my darkest moment."

"If I could give my own life to get her's back, I would," Garnier wrote in the letter read by RCMP Cpl. Jody Allison in the video, as members of Campbell's family watched from the gallery.

"I don't expect you to forgive me for what happened, so I won't ask for your forgiveness... I only hope this will give you some closure."

He also said that he was "sorry this happened."

"I will carry this with me for the rest of my life," Garnier wrote in blue pen on lined paper.

Garnier sat quietly at his lawyers bench, watching himself on a computer monitor as the video was played over several court days. His loved ones could be seen crying at points of the interview.

Under cross-examination by lawyer Joel Pink, Allison agreed that he told Garnier during the interview that he would not lie to him, and that he was being truthful when he said things like "You're a good guy" and "You made a mistake."

Pink noted that Allison said the word "mistake" 16 times over the course of the interview, that he used the phrase "you're a good guy" 16 times, and that he used the word "monster" 18 times. He also noted Garnier exercised his right to remain silent 64 times. Allison did not dispute those numbers.

Also Thursday, the jury heard the Crown and defence agreed that a gym tag and car keys found in a garbage bag in a dumpster across the road from the McCully Street flat were Campbell's.

On Wednesday, the jury heard that Garnier told officers Campbell was dead when she went into the compost bin, and that he knew that to be true because she wasn't moving or breathing.

He recalled being in the yard of the McCully Street apartment after she stopped breathing, but didn't remember putting her body in the green bin, or walking with it through the city's north end towards the Macdonald Bridge.

He remembered roughly where the body was left, he told police. Garnier was arrested after driving by the area where her body was found, the jury has heard.

The trial continues Monday.