The first time a Suffolk detective met a young Huntington Station man while investigating the October 2014 death of a neighbor, the man seemed helpful, the detective testified Wednesday in Riverhead.

“He was calm and he was cooperative,” Det. Michael Mahan said of Adam Saalfield, now 22. Mahan said he spoke to him briefly while going door to door to ask people if they saw or heard anything at about 11 p.m. two nights earlier, on Oct. 12, when a man stabbed Maggie Rosales, 18, twice in the neck and left her dead on Lynch Street.

Saalfield was arrested less than two weeks later. He is on trial, charged with second-degree murder, before state Supreme Court Justice John Collins.

But on that first day, Mahan said during questioning by Assistant District Attorney Raphael Pearl, he had no reason to suspect Saalfield, who told him he drove to a nearby gas station at 10:45 p.m. the night of the killing to get cigarettes and otherwise didn’t go out.

Sure enough, Saalfield is clearly visible on surveillance video at that time at the gas station. He drove there in his 2000 Nissan Altima, left the car door open while he went inside for a minute, returned and left.

It was days later that detectives noticed similarities between Saalfield in the gas station video and the man in the video of Rosales being killed about 15 minutes later across the street. Using still photos from both videos, Mahan showed jurors that both Saalfield and the killer were about the same height and both wore dark pants, and a dark jacket over a light hooded sweatshirt.

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In the video of the killing, the attacker’s face isn’t visible because the hood is over his head.

Mahan noted that in the gas station video, Saalfield had regular length hair. By the time he met him two days later, he said his head was shaved. His hair is now shoulder-length.

Shortly before a pedestrian discovered Rosales’ body a half hour after she was killed, the same surveillance camera showed a car that looked like Saalfield’s Altima roll slowly by the body as it approached Depot Road.

During cross-examination by defense attorney Craig McElwee of Hauppauge, Mahan conceded, “There are numerous light colored vehicles in the area.”

Pearl said in his opening statement that he doesn’t know why Saalfield killed Rosales, who lives a few doors away from him on the same street. The attack is swift on the video, with no hint of a sexual motive. Mahan said none of Rosales’ clothes were torn or disturbed.

The detective suggested robbery wasn’t a motive either. Rosales’ cell phone was left in the pool of her blood, and her pocketbook was left as well.

The next time Mahan went to Saalfield’s house, it was Oct. 29 and he had a search warrant. After a police dog trained to find cadavers indicated something was in Saalfield’s closet, Mahan said investigators found a pair of black boots.

“They appeared to have a couple of red stains on top of the right boot,” he said. Pearl said DNA testing later showed the stains were Rosales’ blood.