NEPTUNE CITY — Liam McAtasney's mother does not believe her son is capable of killing anyone, including Sarah Stern.

The 21-year-old former classmate of Stern was found guilty of her murder by a Monmouth County jury on Tuesday. Stern, 19, went missing in December 2016 and her car was discovered on the Route 35 bridge between the Neptunes and Belmar.

The most damning evidence against McAtasney was a video in which McAtasney gives chilling, emotionless detail about how he killed Stern for money and then dumped her body with the help of her friend.

In the video, McAtasney calmly recounts to filmmaker friend Anthony Curry how he killed Stern, a classmate he says he only pretended to care about 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.

(Michael Stern)

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.

McAtasney's lawyer, Carlos Diaz-Cobo, tried to make a case that Stern ran away because no body was found and said the video was an audition for a part in a movie. The attorney said Curry is trying to make a name for himself by getting involved in the Stern case.

Interviewed for an ABC News "20/20" two-hour special,  Megan McAtasney said she thought her son's words in the video were only an act.

"Liam doesn't speak about things lightheartedly. That was not Liam to me describing an actual murder. That's not the tone he uses about something serious. It was an act," she said.

"With every bone in my body I do not believe Liam is capable of killing Sarah Stern who he loved and adored. I do not think Liam is capable of killing anybody," she said on the show, which aired Friday.

Up until her son was charged, McAtasney's mother said she thought Sarah Stern had run away.

"We grew up at the beach and when somebody goes in the water they wash up somewhere. And when Sarah didn't wash up anywhere it gave us hope that maybe she had run away," she said.

Megan McAtasney thought that Stern was running away because she had been moving her belongings into the homes of trusted friends. Her son had told investigators that Stern had thought about moving to Canada and that she had previously entertained suicidal thoughts and had a rocky relationship with her father, who was on vacation in Florida at the time of her disappearance.

"All these things were making sense and for some reason she felt she couldn't tell any of us," McAtasney's mother said. "That's what I was thinking and that's why I didn't understand why my son was being charged."

It was the video — recorded in Curry's car — that helped crack the case. Curry went to police after he realized that the missing person's case was eerily similar to a movie idea that he said McAtasney had previously shared with him. Investigators installed the hidden camera in Curry's car and enlisted him for a real-life role of a "Donnie Brasco"-type of  informant.

Using McAtasney's own words on film, investigators confronted Taylor and quickly got him to confess. Taylor took investigators on a tour of Stern's home, explaining how he dragged her body out of the house and helped pinpoint the location on Sandy Hook where McAtasney had buried Stern's money.

With the tape and Taylor's confession, police arrested McAtasney.

On "20/20," Megan McAtasney became emotional when she said Stern's disappearance affected her family.

"Poor Mike Stern doesn't know where his daughter is. Everybody's affected by this. This is a horrible, horrible, horrible situation. And we still can't find Sarah. I wonder where Sarah is. We just stopped looking," she said, burying her face in her hands.

Stern's father does not share her hope that his daughter was still alive, describing Liam McAtasney to the "20/20" correspondent as a "sociopath" and "evil."

McAtasney was found guilty of all seven counts with which he was charged: first-degree murder, first-degree felony murder, first-degree robbery, second-degree desecration of human remains, second-degree conspiracy to desecrate human remains, and second-degree hindering apprehension.

He faces life in prison with no chance of parole at his sentencing on May 24.