The man accused of murdering 19-year-old Sarah Stern has reportedly confessed to killing the young woman to a friend  — who surreptitiously recorded the conversation.

According to NBC New York, prosecutors revealed the bombshell discovery at a hearing on Tuesday. In the recording, suspected murderer Liam McAtasney, 19, reportedly told a friend that he killed Stern in her home on December 2, strangling her with such force that he lifted her off the ground. He then allegedly said he timed how long it took her to die. According to NJ.com, it took 30 minutes.

“He knew exactly how long it was because he chose to time it,” Assistant Monmouth County Prosecutor Meghan Doyle reportedly said.

McAtansey was arrested earlier this month, accused of strangling Stern and conspiring with a friend to dispose of her body, throwing it off the Route 35 bridge in Belmar, New Jersey. Both men were former classmates of Stern’s, and the alleged accomplice, Preston Taylor, 19, reportedly took Stern to prom.

A possible motive for the murder is robbery: McAtansey reportedly knew of a large cash withdrawal Stern had made shortly before her death. NBC New York reports that thousands of dollars in cash were stolen from Stern, and some of the cash was later found in a safe in a Neptune, New Jersey, park.

Stern’s body has yet to be found.

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