Then, one by one, police say he stabbed to death four men and one woman in what authorities call the worst mass murder in Calgary’s history.

“We have never seen five people killed at one scene,” Calgary Police Chief Rick Hanson said. “The scene was horrific.”

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PICTURE: Lone suspect in Brentwood murders is reportedly Matt de Grood. His father is police Inspector Doug de Grood. pic.twitter.com/N2njYGrfpo — 660News Calgary (@660News) April 15, 2014

Police are confronted by a confounding set of circumstances and no motive. De Grood did not appear to be inebriated when he was arrested. “Was there anything that precipitated the event?” Hanson said. “Was there something that anyone had done that anyone could have taken as an insult or an affront to this individual? To the best of our knowledge right now, there’s nothing to indicate anything like that happened.”

Neighbors told The Associated Press the youths had gathered around a fire pit Monday night and, while drinking beer, talked about politics and the stock market. Then, at 9 p.m., the neighbor said, the twentysomethings disappeared into the house, and he didn’t hear from them again. De Grood, who was charged Tuesday with five counts of murder and doesn’t have a criminal record, arrived after midnight that night.

At 1:20 a.m., police got a call from someone inside the house. There had been a stabbing.

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“[He allegedly] arrived at the party, obtained a large knife and targeted the victims one by one, stabbing them several times,” Hanson said. When police arrived, three men were dead inside. A fourth man and a woman were discovered bleeding on the lawn. Police discovered de Grood a few blocks away and set several dogs after him. The dogs brought him down.

This is a photo of the suspect in the mass homicide from overnight. The son of a CPS officer #yyc pic.twitter.com/uHxfL5feiC — Nancy Hixt (@NancyHixt) April 15, 2014

The Calgary Herald identified the victims as Lawrence Hong, Josh Hunter, Kaitlin Perras, Zackariah Rathwell and Jordan Segura.

“These were all good kids,” Hanson said. “There’s no question about that. They did nothing wrong and nothing that they did contributed to what happened to them.”

Friends deluged Twitter with both sadness and dismay. “I remember looking up to Zack [Rathwell] so much in elementary and that carried over,” read one tweet. “I still can’t believe that it was you last night! It was a ton of fun drumming with you,” said another, about Josh Hunter.

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Indeed, a few additional clues about the alleged murderer emerged on social media. Wednesday morning, it was still unclear whether de Grood’s apparent affinity for Megadeth is linked to the murders, but it wouldn’t be the first time causality has been drawn between music and violence.

Picture of Matt de Grood from 2008-09 St. Francis yearbook. Expected to be charged with 1st-degree murder #cbc pic.twitter.com/ABcDedQ9At — Devin Heroux (@Devin_Heroux) April 15, 2014

Jared Loughner, who killed six people and injured 14 more, including congresswoman Gabrielle Giffords, in a Tucson parking lot, was a fan of a heavy metal song called “Bodies.”

Months later and hundreds of miles away, police discovered a metal album inside a burned house where one youth had killed his grandparents.

Beyond de Grood’s Megadeth reference, there was no other mention of music on his profile, since taken down.

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Other postings were substantially more benign. Last December, the University of Calgary psychology student announced his acceptance to law school. After 49 people “liked” the update and 11 congratulated him, he wrote, “Thank you all very much for the support!” He also uploaded an image of a capital “E” against a black background and wrote, “Equal rights for all races & identities under this charter.”

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His “likes” included Hinduism, Hamlet, sociology, anti-drug abuse, and cognitive behavior theory. He was also a fan of motivational speaker Chris Gardner and Jonathan Haidt’s “The Righteous Mind: Why Good People are Divided by Politics and Religion.”