A teenager in Massachusetts has been charged with murder after he allegedly stabbed his 16-year-old classmate and cut his head off in what the district attorney is calling a "horrific, horrific murder."

Over the weekend, 15-year-old Mathew Borges was arrested and charged with the murder of his 16-year-old classmate Lee Manuel Viloria-Paulino, whose headless body was found last week near the Merrimack River in Lawrence, Massachusetts, according to the Associated Press. The boy's head was found nearby. Borges will be charged as an adult for first-degree murder.

Just days before his death, Viloria-Paulino had introduced Borges to his family for the first time. Viloria-Paulino was last seen leaving his grandmother's house in Lawrence on Nov. 18. That night, according to Fox 25 (WFXT), Viloria-Paulino and Borges went to the river to smoke weed and watch a boat house light up on the river at night, as they both enjoyed doing.

Then, Borges allegedly stabbed his classmate and mutilated his body so severely that the autopsy took 11 hours, according to the Boston Globe. Essex County District Attorney Jonathan Blodgett called it a "horrific, horrific murder" which "shook the community." 

A witness told police that Borges confessed to him. The police report states: "A witness told investigators that Mathew told them he did something bad. 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."

Borges' motive is not clear at this point.

Fox 25 reports that Borges pled not guilty on Monday and is now being held without bail. He is due in court again in January.