Moments before a Huntington Station teenager’s lifeless body was found in the street, the man later charged with second-degree murder was seen driving from that area by his mother’s boyfriend at the time — a Suffolk police detective.

The detective, Gene Geissinger, testified Friday that he was taking Renee Saalfield to her house in Huntington Station after three days away when he noticed her son, Adam Saalfield, 22, driving north on Depot Road.

During questioning by Assistant District Attorney Sheetal Shetty, Geissinger said he followed behind for a few blocks and they all arrived together at the home the Saalfields shared about 11:25 p.m. on Oct. 12, 2014.

Less than a half-hour earlier, Suffolk prosecutors say Adam Saalfield grabbed his neighbor, Maggie Rosales, 18, from behind as she walked on nearby Lynch Street, stabbed her twice in the neck and left her dead in the street. Not long after Geissinger and the Saalfields got to their house, a pedestrian found Rosales’ body.

Adam Saalfield is on trial before state Supreme Court Justice John Collins in Riverhead.

Although it was a chilly night, Geissinger said Saalfield was dressed in shorts and a T-shirt. Nothing seemed amiss, and he said Saalfield helped him unload his truck.

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“He was absolutely Adam,” Geissinger said, adding that he had had a good relationship with Saalfield.

Geissinger said Saalfield was wearing different clothes — a black leather jacket over a light hooded sweatshirt — when he was spotted on video at a gas station about 10:45 p.m. on Oct. 12.

“It’s Adam, 100 percent,” Geissinger said of the video, noting that Saalfield often wore those clothes.

The man seen attacking Rosales just before 11 p.m. in a surveillance video was dressed the same, but the attacker’s hood was up and hid his face. When the same video continued, it showed a silver 2000 Nissan Altima roll slowly past the body about 11:25 p.m. Geissinger said he saw Saalfield’s silver Altima come from that direction as he was turning on to Depot Road from East 12th Street.

During cross-examination by defense attorney Craig McElwee of Hauppauge, Geissinger said Saalfield also might have been coming from a 7-Eleven farther south on Depot Road, and that Saalfield often went there for junk food.

Earlier Friday, Suffolk Police Officer Keith Mahoney said he arrested Saalfield on a drug charge Oct. 18, 2014, in front of his house. During the arrest, Mahoney said he seized a black KA-BAR police knife, which Saalfield kept in a sheath near his right hip. The blade was 2.3 inches long, he said.

Mahoney said he noticed and photographed several cuts, including one on Saalfield’s cheek and several on his right arm.

During questioning by McElwee, Mahoney said he was part of increased patrols in the area because of three killings in the area recently. McElwee suggested in his opening statement that police were eager to make an arrest because of community pressure.

After Saalfield’s arrest was processed, Mahoney said he did not give the knife back to him.

“Why did you keep a legal knife?” McElwee asked.

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“I did it at the direction of the Homicide Squad,” Mahoney said.