FREEHOLD -- A former classmate told a friend he strangled Sarah Stern with such force that he lifted her off the ground and then watched for 30 minutes as she died in her home, prosecutors told a judge Tuesday morning.

Liam McAtasney, 19, actually watched the clock as Sarah Stern took her last breaths, Assistant Monmouth County Prosecutor Meghan Doyle told Superior Court Judge David Bauman during a detention hearing.

"He knew exactly how long it was because he chose to time it," Doyle said.

The new revelation in the killing of Sarah Stern, whose body has not been recovered, came as prosecutors succeeded in convincing Bauman that McAtasney, of Neptune City, should not be released from jail while his case is pending.

McAtasney gave the description of the killing to a friend, who secretly videotaped a meeting between the two in which McAtasney provided details of the strangulation, Doyle said.

McAtasney's attorney, Charles Moriarty, did not fight the prosecution's attempt to keep his client behind bars, but after court, he insisted McAtasney is innocent.

"He didn't hurt her and wouldn't hurt her," Moriarty said.

Authorities on Feb. 2 charged McAtasney and his friend Preston Taylor, also 19, in the Dec. 2 disappearance and death of Stern, their 19-year-old friend who grew up with them in Neptune City.

After the hearing, McAtasney's attorney denied his client played a role in Stern's death, instead suggesting the defense's strategy may be to show Stern was recently troubled and may have taken her own life or even that she is still alive.

"I hope for her family that the young woman is still alive," he said.

He said information he received indicated that Stern "was depressed" and "had some problems." On the day she disappeared, he said, she handed out "all her mother's personal possessions" to friends and relatives.

Monmouth County Prosecutor Christopher Gramiccioni would not comment on Moriarty's suggestions.

Doyle said McAtasney told the friend that while strangling Stern with his hands, he lifted her off the ground and then left her body on the floor while he watched her die.

She said he demonstrated to the friend how he lifted her into the air and that he planned for weeks to rob her.

In arguing for McAtasney's continued incarceration, Doyle said he tried to cover up the killing and his role in it.

"For the two months after that, this defendant spent the entire time calculating every one of his moves to continue to cover up the murder and death of Sarah Stern," she said.

She said McAtasney lied to police in a December interview with them and that he participated in a search with Stern's family and friends.

"He thought that that would alleviate his guilt," she said.

She said he also mischaracterized his relationship with Stern to authorities.

"He pretended to make their friendship more than it was so people would not focus on him as a suspect," Doyle said.

Moriarty said he didn't fight for McAtasney's release on Tuesday in part because he had only recently received evidence from the prosecutor's office.

He called into question the reliability of the video recording between his client and the witness and Taylor's statement to investigators. He said they sounded more "scripted" than a supposed recollection of events.

"The videos are disturbing because both boys are talking about something that didn't happen," Moriarty said of McAtasney and the witness. "It's unusual, people talking like this, like they're talking about going to dinner."

He called it "lockerroom talk."

He wouldn't say under what circumstances the recording was made with the witness but said it took place in a car. Bauman had previously noted that the recording was made on Jan. 31.

Stern's car was found on the Route 35 bridge over the Shark River in Belmar early Dec. 3, sparking a search later that day of the river.

Prosecutors contend McAtasney strangled Stern in her home before robbing her of thousands of dollars. They say he left her there for eight hours before he and Taylor returned to the house to retrieve her body and dump it over the side of the bridge.

McAtasney is charged with murder, felony murder, robbery, conspiracy, disposal of human remains and hindering apprehension.

Taylor is charged with conspiracy, disposal of human remains and hindering apprehension.

On Feb. 7, Bauman ordered Taylor to remain behind bars until his trial.

During Taylor's detention hearing, prosecutors alleged that McAtasney planned for six months to rob and kill Stern and that he ordered Taylor to go to Stern's home after the robbery and remove her body from the house.

Taylor hid Stern's body under a bush at her house before McAtasney and Taylor retrieved it, prosecutors said. Followed by Taylor in his own car, McAtasney drove her body in Stern's car to the Route 35 bridge in Belmar where they tossed her into the Shark River, prosecutors have said. The search for Stern spanned months after her car was found abandoned.

Investigators recovered two safes, one in Shark River Park and one in Sandy Hook, containing cash and articles of Stern's clothing, prosecutors said. Taylor is the one who led detectives to the two safes, authorities said.

Prosecutors still have not said what led detectives to suspect McAtasney and Taylor were behind Stern's disappearance. The two grew up with Stern and went to Neptune High School with her. Taylor and Stern went to junior prom together.

Stern was considered a missing person until the Monmouth County Prosecutor's Office announced it had made a break in the case and arrested McAtasney and Taylor.

The search for her body has been suspended since last week because of weather and water conditions.

MaryAnn Spoto may be reached at mspoto@njadvancemedia.com. Follow her on Twitter @MaryAnnSpoto. Find NJ.com on Facebook.