The end came quietly, swiftly and brutally for a Huntington Station teenager, as shown in a video of her killing that was played Monday in a Riverhead courtroom.

The video of the death of Maggie Rosales, 18, was shown during the first day of the trial of Adam Saalfield, 22, her neighbor on Leyden Street. He is charged with second-degree murder.

The surveillance video came from a camera on the side of Sinai Furniture, a store at the corner of Lynch Street and Depot Road. It shows Rosales walking west on the left side of Lynch Street shortly before 11 p.m. on Oct. 12, 2014. About two steps behind her is a man in a light hoodie and dark jacket.

Rosales, a senior at Walt Whitman High School in South Huntington, is oblivious, listening to music from ear buds plugged into her phone as she walked a few blocks to see her cousin and some friends, Assistant District Attorney Raphael Pearl told jurors in his opening statement before state Supreme Court Justice John Collins.

Without warning, the man caught up to Rosales, grabbed her neck from behind and pulled her to the ground. He was on top of her briefly, pulled her to her feet and then they fell to the ground again, next to a parked Mitsubishi sport utility vehicle. Rosales appeared unable to defend herself.

The attacker was Saalfield, Pearl said. Saalfield slashed the much smaller Rosales twice,cutting through both carotid arteries, a jugular vein and her trachea, Pearl said. He even left a cut in one of the bones of her neck, he said.

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Several members of her family wept quietly as the video played, as they did earlier when Pearl described the killing.

“He left her on the side of the road, choking and gasping on her own blood on the side of the road,” Pearl said. “That was where Maggie Rosales’ life ended, cold and alone on that cold, October night.”

About 15 minutes before that, a video from a nearby gas station showed an identically dressed man getting some cigarettes. But in this video, the man’s hood was down and he resembled Saalfield.

Pearl said investigators found a trail of blood, a quarter of a mile long, leading from the body.

“He left a blood trail from where he murdered Maggie Rosales, right back to his house,” Pearl said. “That blood trail extends right to his bedroom.”

A search warrant found boots in Saalfield’s closet that had Rosales’ blood on the top of them, Pearl said. Some of the blood on the trail had DNA from both Rosales and Saalfield, he said.

“I won’t tell you why this defendant chose to murder Maggie that day,” Pearl said, noting that motive is not an element of a murder charge.

Defense attorney Craig McElwee of Hauppauge told jurors the proof against his client is not as strong as Pearl suggested.

“You cannot tell the identification of the individual” who attacked Rosales in the video, he said. Like Pearl, he said his client never had any kind of relationship with Rosales, even though there were neighbors. He had no reason to stalk her and slit her throat, McElwee said.

He noted that in the 12 days between Rosales’ death and Saalfield’s arrest, there were claims in the community that police didn’t care about the deaths of minorities in Huntington Station.

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“The community was outraged,” McElwee said. “The police were under tremendous pressure to do something.”

Rosales’ blood on Saalfield’s boot 12 days later doesn’t mean he killed her, McElwee said.

In testimony, Donald Ulloagueso, 20, said he was walking along Depot Road when he spotted the body on Lynch Street. He thought someone had passed out and was going to offer his sweater, but then saw blood and called 911.

When paramedics came, they rolled the body onto her back and Ulloagueso said there was so much blood he couldn’t recognize who it was. It was only later that he found out it was Rosales, a friend of his from school.

Rosales’ father, Cesar Rosales, testified that she left the house just before 11 p.m. to see friends on the night before Columbus Day. He described the clothes she was wearing — the same as in the video of her death.

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He was at work in the kitchen at Huntington Hospital when the next morning when detectives showed up.

“They gave me the bad news,” he said.