The mother of a New Jersey man who was convicted on Tuesday of the murder of his childhood friend Sarah Stern said she doesn't believe her son could have killed anyone.

"I do not believe that. In every bone of my body, I do not believe that Liam is capable of killing Sarah Stern, who he loved and adored," Megan McAtasney told "20/20" in an exclusive interview before her son was found guilty of first-degree murder.

Watch the full story on "20/20" TONIGHT at 9 p.m. on ABC.

Liam McAtasney, now 21, was found guilty on all seven counts against him, including first-degree murder, conspiracy, desecrating human remains and other charges. He was accused of strangling 19-year-old Stern, his high school classmate and childhood friend, in December 2016.

Along with Preston Taylor, Stern's junior prom date and Liam McAtasney's former roommate, he threw her body off a bridge between Neptune and Belmar, New Jersey.

Taylor pleaded guilty to helping dispose of Stern's body and agreed to testify against McAtasney. Taylor also pleaded guilty to first-degree robbery, second-degree conspiracy to commit robbery and second-degree disturbing or desecrating human remains. Authorities say McAtasney killed Stern in order to steal a large sum of money left by her late mother. Taylor admitted to his role in the theft.

Both are expected to be sentenced in May.

Stern's body was never found, but police discovered her abandoned car on Dec. 3, 2016, on the Route 35 Bridge between Neptune and Belmar with the keys in the vehicle.

Megan McAtasney said her son and his twin brother, Seamus, first met Stern during Sunday school classes when they were about six years old. They became friends in middle school.

"There was just an instant connection," she recalled. "They had a nickname for their group called 'the Squad,' and the Squad would always come up with the weekend plans. And then the moms would try to figure out how we were going to get the Squad to all of their weekend plans, whether it was Six Flags or the movies or whatever adventure they had decided they're doing that weekend."

Liam was close with Stern, and his family was there for her when Stern's mother died in 2013, Megan McAtasney said.

In December 2016, the mother said she learned Stern was missing when police officers arrived at her home in the early morning.

"The police ask me if Sarah's at my home, and I tell them no. And they said they can't find Sarah. And I tell them, 'Not a problem. I'll call her. She'll answer for me,'" she said.

She said the call went to Stern's voicemail, and police then asked if Liam was home. She said she told them he lived at a different address, and that she went back to bed thinking Stern might be asleep at his apartment.

The next morning, she said she learned police found Stern's car abandoned on a bridge and they believed she might have jumped off the bridge.

"I just started screaming," McAtasney said. "And the police were asking me if I had seen Liam, and then I broke down again because I thought that Liam was missing, too. And they assured me that they had seen Liam in the middle of the night when I directed them to go to Liam's house."

She said she eventually ran into Liam and Taylor while driving and told her son police believed Stern may have jumped from the bridge and that they wanted to speak to him again.

"We both just started crying," she said.

However, in January 2017, detectives with the Monmouth County Prosecutor's Office set up a sting operation and secretly recorded a conversation between Liam McAtasney and his friend Anthony Curry, which authorities say was a confession by McAtasney to murdering Stern.

In the video of the conversation, McAtasney was recorded telling Curry he choked Stern to death.

"She just p----- 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 her out and have her out in like a couple of minutes," McAtasney is heard telling Curry in the recorded video.

Carlos Diaz-Cobo, Liam McAtasney's defense attorney, called the statements a "false confession." According to Diaz-Cobo, McAtansey was auditioning for Curry, who is a filmmaker.

"This statement was made to an individual that he admired. Somebody that was, or, at least has engaged in the business of filmmaking. More specifically, this particular individual, engaged in horror filmmaking. And so, that's something that appeals to Liam," Diaz-Cobo said in an interview with "20/20" before McAtasney was found guilty. "He wanted to be a part of that filmmaking process, and be in this individual's world. And so, he tends to make up things in order to make himself be either tougher than he is or different than who he actually is. And I think that's something that is characteristic of Liam."

Diaz-Cobo said there's no evidence that supports the statements McAtasney made to Curry in the video recording of their conversation.

"There's no evidence, physical evidence that there was any vomit, urine, blood found in that house. And so, that lack of evidence tells me that the false statement or false confession that he gave Anthony Curry is absolutely false," he said.

Two months after Stern was last seen, McAtasney and Taylor were arrested and charged with her murder. Megan McAtasney said she was in disbelief when she learned her son was charged with murder.

"I grew up at the beach, and when somebody goes in the water, they wash up somewhere. And when Sarah had not washed up onto any of our shores, it gave us hope that Sarah had maybe run away. And so, in my head, I was thinking that Sarah needed to get away and for some reason she felt she couldn't tell any of us," she said.

McAtasney, who also has a younger daughter and another son, said Stern had previously told her daughter of her desire to move away from New Jersey.

"She had expressed that to my daughter that she wanted to move to either Canada or California, and Liam told me that that was something that she was going to be doing in the future," she said.

The possibility that her son could be involved "did not make any sense" to McAtasney.

"We were thinking that my son was set up because this was not anything that anybody would have expected from Liam. Nothing made sense at that point and time. Nothing makes sense at this point and time," she said.

"I mean, everybody is affected by this. This is a horrible, horrible, horrible situation. And we still can't find Sarah. We don't know where Sarah is. And we just stopped looking," she added.

Liam McAtasney is facing a life sentence without the possibility of parole but continues to maintain his innocence.

"He regrets making statements, making things up. He regrets it all. But most importantly, I think that he regrets that Sarah's missing," Diaz-Cobo said.

His attorney says they plan to file an appeal.

"My client and his family are disappointed in the verdict. ... It was difficult, especially for a 20-year-old boy and his loving mother and family. Their only solace now is that there are several issues for appeal which we intend to file at the appropriate time and believe will reverse this conviction," Diaz-Cobo said in a statement provided to ABC News after the verdict.

"I know where my son is. My son is in jail," Megan McAtasney said. "Michael Stern doesn't know where his daughter is. It's very hard."

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