LAWRENCE, Mass. — A Massachusetts teenager recounted to a witness how he stabbed his 16-year-old classmate before cutting off his head, police say, after the two students had taken a walk together to smoke marijuana.

Mathew Borges, 15, was arraigned Monday morning for the first-degree murder of Lee Manuel Viloria-Paulino, 16. The two were students at Lawrence High School.  

“A witness told investigators that Mathew told him he did something bad,” according to a report by Lawrence Police Detective Jay Heggarty.

“Mathew then told him that he stabbed a kid and cut his head off, killing him. When Mathew said this he was motioning with his hands as if he was stabbing someone and cutting someone’s head off. Mathew then walked out of the house,” Heggarty wrote in his report. 

Vilora-Paulino disappeared Nov. 18 from his home in Lawrence, a riverside city of 77,000, leaving his iPhone and wallet in his room. He had been missing for nearly two weeks when a woman walking her dog called police saying she’d found a decapitated body in the river not far from a dead-end street.

Investigators found the head on the riverbank about 50 feet away, behind the neighborhood Boys & Girls Club. Police released Vilora-Paulino’s identity on Dec. 2. 

Wearing a green T-shirt with his curly hair hanging down over his eyes, Borges showed no emotion as he stood in the prisoner’s dock for the roughly 90-second arraignment Monday.

Borges’ next court appearance was set for Jan. 10.

Police obtained a video showing Viloria-Paulino leaving his house with a friend who was later identified as Borges, according to Heggarty’s report. 

Borges also confirmed to police that it was him in the video, Heggarty noted. 

He told police that he and Viloria-Paulino went to walk along the Merrimack River “to smoke marijuana.” He described to Heggarty the route they took to get to the river and where they smoked pot. 

“Mathew said they both like nature and that they like the way the boat house across the river is lit up at night,” Heggarty wrote. 

He told Heggarty he left Viloria-Paulino there at 7 p.m. and he walked home.

“This was the last time that Lee was known to have been seen alive by another person,” Heggarty wrote.

Harmacinski writes for the North Andover, Massachusetts Eagle-Tribune.