A 15-year-old charged with murder told a friend he stabbed his classmate and cut his head off, according to court documents.

In a police report obtained by WFXT, investigators said Matthew Borges, 15, was the last person to see Lee Viloria-Paulino alive. Borges allegedly told police he and Viloria-Paulino, a classmate at Lawrence High School, had gone to Merrimack River to smoke marijuana on the evening of Friday, Nov. 18.

But Borges said he left Viloria-Paulino there and went straight home, according to police.

Borges told police he and Viloria-Paulino liked the way the boat house on the river lit up at night and that's why they went to smoke there. Investigators said Borges told them he left Viloria-Paulino there at 7 p.m. that evening and Viloria-Paulino wasn't seen again until his decapitated body was found along the river on Dec. 1.

Police said a witness, however, told them about an encounter with Borges, in which he confessed to killing Viloria-Paulino.

"Matthew then told him that he stabbed a kid and cut his head off, killing him," the report stated. "When Matthew said this, he was motioning with his hands as if he was stabbing someone and cutting someone's head off."

Borges was arrested on Saturday and has been held on $250,000 cash bail. He is facing a murder in the first-degree charge and is being charged as an adult, said the District Attorney's Office.

He pleaded not guilty Monday morning and is due back in court in January.