Liam McAtasney, now 20, allegedly killed 19-year-old Sarah Stern in December of 2016 and disposed of her body off a bridge in Belmar.
His friend Preston Taylor pleaded guilty to helping dump her body and has agreed to testify against McAtasney.
McAtasney's attorney, Charles Moriarty, claimed Wednesday that his client made up stories about planning to rob and kill Stern as part of a horror film an acquaintance was making.
Moriarty told state Superior Court Judge Richard English that the acquaintance told detectives that McAtasney was "always making things up to be more interesting." Police say the acquaintance secretly recorded McAtasney confessing to the crime, and Moriarty is seeking to have that recording excluded from evidence at McAtasney's upcoming trial.
Related: Memorial service held for New Jersey teen Sarah Stern
Monmouth County prosecutors say the Neptune City man killed Stern during a robbery that netted $10,000.
Stern was last seen at her home on the night of December 2. Hours later, her unoccupied vehicle was found on the Route 35 Bridge in Belmar over the Shark River Inlet with the keys still in the ignition. Her body has never been found.
The state is seeking a life sentence without parole in the case against McAtasney, who also faces charges of robbery, conspiracy, desecration of human remains and hindering apprehension. He has pleaded not guilty.
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(The Associated Press contributed to this report)