On Thursday came a gruesome discovery: A woman walking her dog found Viloria-Paulino’s headless body on the banks of a river, 30 miles north of Boston.

Mathew Borges, the mysterious classmate, was charged with murder two days later. Borges, 15, will be tried as an adult in the killing, Essex District Attorney Jonathan Blodgett told the Associated Press.

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Investigators rushed to reassure frightened residents in the Tower Hill community that the killing was not a random attack, even as they turned the banks of the Merrimack River into a crime scene.

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Nearby, they found the teen’s severed head.

The grisly killing sent shock waves through the community.

“This is not the only thing that has happened in my district,” City Councilor Jeovanny Rodriguez told public radio station WBUR. “Multiple shootings throughout the summer. A lot of people are concerned in terms of safety. I have faith in the police department, but people are concerned and very scared.”

Hundreds of people gathered Friday at a memorial in the teen’s memory. On Saturday, a short time before authorities announced the arrest, Viloria-Paulino’s family made its first public statements about the killing.

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“I can describe him only as a lovely kid that hasn’t done nothing bad to no one,” his grandmother, Ivelisse Cornielle, said at a news conference before the arrest was announced.

Cornielle told the Boston Globe that police initially dragged their feet when the family reported the teen missing. If they had taken the family’s worries seriously sooner, she said, her grandson might still be alive.

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“We’ve been here since the beginning, asking for help,” she said, according to the Boston Globe. “We are poor, we are Hispanic, they considered this a normal runaway case. I told them from day one that it wasn’t.”

Blodgett called the killing a “horrific, horrific murder.”

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“While it shook this community, we were completely and totally committed in our resolve to … bring this matter to justice, and I’m pleased to say today that we were able to do that,” he said.

A police report obtained by WBZ NewsRadio said Borges ultimately confessed to another person.

After he was arrested, Borges told investigators that he and Viloria-Paulino went to a spot on the river that teens frequent. They smoked marijuana and watched a boat house across the river light up, the newspaper reported.

Then, Borges allegedly stabbed his friend and mutilated his body to such an extent that it took half a day to perform an autopsy.

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Later, the report says, Borges told another person what had happened.

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“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.’’

On Monday, relatives of the dead teen crowded into Borges’s arraignment hearing, where he pleaded not guilty and was ordered held without bail.

But one of the most pressing questions remained unanswered: Why?

A previous version of this post said Mathew Borges was 16. He is 15. The post has been updated.