LAWRENCE, Mass. (AP) — 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.

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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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