EDITOR'S NOTE: This story contains graphic descriptions of violence.

FREEHOLD BOROUGH — A Monmouth County jury watched a secretly videotaped confession in which a man accused of strangling a former high school classmate in 2016 describes how he killed her and timed her death.

NJ.com reports the conversation between Liam McAtasney and his friend occurred in the friend's car on Jan. 31, 2017, days before McAtasney and an accomplice were charged in 19-year-old Sarah Stern's death.

In cold-blooded detail, McAtasney calmly recounts how he killed Stern, a classmate he says he only pretended to befriend but in reality had spent half a year planning her demise. He also describes how he and Preston Taylor, who went to the high school prom with Stern, took her body from her Neptune City home and dumped it into the Shark River Inlet in December.

Stern's body has never been recovered, but Taylor has testified that he helped McAtasney throw the body over the bridge. Taylor has pleaded guilty to first-degree robbery, second-degree conspiracy to commit robbery, and second-degree disturbing or desecrating human remains.

Defense lawyers say McAtasney, 21, made up the entire story on the video because he wanted to impress the friend who recorded it, an experienced horror movie maker.

Liam McAtasney, right, listens to his defense attorney Carlos Diaz-Cobo during his murder trial at the Monmouth County Courthouse in Freehold, N.J., Thursday, Jan. 24, 2019. McAtasney is accused of strangling Sarah Stern, a former high school classmate, during a robbery and throwing her body off a New Jersey bridge in December 2016. (Russ DeSantis/NJ Advance Media via AP, Pool)

Stern's father and family members cried as the video played in court Thursday.

The Asbury Park Press published long excerpts from the video. Warning: The video contains graphic language and content.

McAtasney says in the video he "pretty much hung her, like, I just picked her up and had her dangling off the ground" and it took 30 minutes for her to die, which he knew because he set a timer on his phone.

"She just (urinated) herself and said my name and then that was it. And it took me a half an hour to kill her. I thought I was going to be able to choke and have her out in a couple of minutes," he says.

He says his biggest problem was Stern's dog, which "laid there and watched as I killed her. Didn't do anything ... what kind of dog is that?"

McAtasney expresses no noticeable regret or emotion in the video.

His friend asks him if Stern had screamed.

"I choked her out and she was just laying there having a seizure or something. … I got a shirt and I just shoved it down her throat so she wouldn’t throw up or anything and held my finger over her nose and set a timer."

Preston Taylor testifies for the prosecution during the murder trial of Liam McAtasney at the Monmouth County courthouse Wednesday, Jan. 23, 2019, in Freehold, N.J. McAtasney is accused of strangling a former high school classmate, Sarah Stern, during a robbery and throwing her body off a New Jersey bridge in Dec. 2016. (Patti Sapone/NJ Advance Media via AP, Pool)

The ostensible motive for the killing was $100,000 that Stern had inherited, but McAtasney says he recovered less than $10,000 in cash. He complains in the video that the bills are too old and deteriorated, which he worries might allow police to track him.

McAtasney says he spent months observing Stern, saying that on the night of the killing, he backed Stern's car out of the garage in the same way he had seen her do it just in case the neighbor's security camera caught the car on video.

"I needed to make it seem that we were better friends than we actually were so they wouldn’t question my behavior with her," he says.

In the video, he describes dumping the body with Preston.

McAtasney says that after he killed Stern, he left her body in the bathroom so that he could go to work. Taylor then went to Stern's house and dragged the body outside, hiding it in bushes, he says. After McAtasney left work, he returned to Stern's home and they placed her body in the passenger seat of her car, making it look like she was still alive, he says.

He says he "underestimated" his strength because he was unable to throw her body off the bridge by himself. He says the plan had been to dump the body and then cross the bridge and get into Preston's waiting car, making it look like Stern had parked her car and committed suicide. Instead, he says, Preston had to make a U-turn and help him throw the body into the water.

"I don’t feel any different and I don’t think about it," McAtasney tells his friend in the video. "I planned it out for half a year. I had patience."

The Associated Press contributed to this report.