Updated

A 15-year-old Massachusetts boy charged with killing a high school classmate whose headless body was found near a river last week is being held without bail.

Mathew Borges was held after pleading not guilty at a brief arraignment Monday on a first-degree murder charge.

Borges is being charged as an adult, so court proceedings were open. His lawyer did not argue for bail.

The body of 16-year-old Lee Manuel Viloria-Paulino was found near the Merrimack River in Lawrence on Thursday by a woman walking her dog. Police recovered his head nearby. He had been missing since Nov. 18.

Borges was arrested Saturday.

Essex District Attorney Jonathan Blodgett says the motive for the killing is unclear and remains under investigation.

Both boys attended Lawrence High School.

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This story has been corrected to show the suspect is 15, not 16, per new information from authorities.