Suspect Seen with Decapitated Teen the Night Victim Went Missing: Police

A motive continues to elude investigators in the recent slaying and decapitation of 16-year-old Lee Manuel Viloria-Paulino, whose headless body was found Thursday, weeks after he vanished, partially submerged in the Merrimack River in Lawrence, Massachusetts.

PEOPLE learns that authorities still cannot say what drove Mathew Borges, 15, to allegedly repeatedly stab and behead Lee, who was one of his classmates. An arrest report alleges Mathew confessed to the crime — but not to police.

A witness allegedly told investigators the teen had admitted he’d done “something bad,” according to the report, which was obtained by PEOPLE.

Specifically, Mathew claimed “he stabbed a kid and cut his head off … motioning with his hands as if he was stabbing someone and cutting someone’s head off,” the report alleges.

Investigators have also obtained surveillance video that shows Lee exiting his home the same night he vanished, according to the police report. Mathew can also be seen in the footage, which was captured the evening of Nov. 18.

In the security video, the two teens allegedly walk off together and out of the frame. Lee was missing two weeks before his remains were discovered by a pedestrian.

During his arraignment Monday, Mathew pleaded not guilty to a single murder count. Hayden Edward, his defense attorney, was unavailable for comment.

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Mathew was arrested on Saturday. According to his arrest report, he told detectives he and Lee liked walking along the Merrimac River while smoking marijuana. Mathew allegedly told police that the two high school sophomores both appreciated nature and “the way the boat house across the river is lit up at night.”

Mathew allegedly stabbed and mutilated Lee’s body so severely that his autopsy took 11 hours, prosecutors said.

“This was a horrific, horrific murder,” Essex District Attorney Jonathan Blodgett said at a news conference on Saturday. “While it shook the community, we were completely and totally resolved to bring this matter to justice.”

Hundreds attended a memorial for Lee on Friday. The next day, Mathew was charged with his murder, and the slain teen’s family held a brief news conference.

“I can describe him only as a lovely kid that hasn’t done nothing bad to no one,” Lee’s grandmother, Ivelisse Cornielle, told reporters. (Lee’s family declined PEOPLE’s request for interviews.)

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Borges is being tried as an adult and is being held without bail. He is scheduled to appear in court on Jan. 10 for a probable cause hearing.

According to various media accounts, Lee’s family is faulting Lawrence police for not taking initial reports of the teen’s disappearance more seriously. They allege that authorities dismissed their concerns, suspecting the teen had instead run away.

Calls and emails to the Lawrence Police Department, seeking comment on these claims, have not been returned.