Drop by drop, investigators followed a trail of blood that led from the lifeless body of a Huntington Station teenager one cold night in October 2014, to an intersection a quarter mile away, they testified Tuesday.

Adam Saalfield, 22, lived at one of the houses at that intersection. He is on trial before state Supreme Court Justice John Collins, charged with second-degree murder in the death of Maggie Rosales, 18, whose body was at the start of the blood trail. He is accused of slashing her throat the night of Oct. 12, 2014.

But before the blood trail was found, an emergency medical technician from the Huntington Community First Aid Squad checked for signs of life, she testified Tuesday.

“She was a young woman lying in a pool of her own blood,” Nicole Calderone said during questioning by Assistant District Attorney Sheetal Shetty. “It was a significant amount of blood — probably the most I have ever seen.”

In a crime scene photo, the pool of blood extended from Rosales’ head to below her knees.

Calderone said she and her partner rolled Rosales, who was lying face down in the blood, onto her back. It was obvious she was dead, Calderone said, so she checked on Donald Ulloagueso, the young man who had found the body lying on Lynch Street, just east of Depot Road. “He’s beside himself,” Calderone said.

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The attack was captured on surveillance video from the furniture store at the corner. On Monday, jurors watched the video of a man with a hooded sweatshirt over his face walk behind Rosales, grab her neck from behind and pull her to the ground.

Police Officer John Whidden testified that he put together a map that diagrammed the location of 59 bloodstains over a path that extended 1,512 feet. The trail started next to Rosales’ body and then headed east on Lynch Street, away from Depot Road, on the right side of the street before crossing over to the left.

In his opening statement Monday, Assistant District Attorney Raphael Pearl said a few of the drops contained DNA from both Rosales and Saalfield.

At the end of the block, the trail turned left to go north on Varney Avenue.

Forensic scientist Thomas Zaveski of the Suffolk Crime Laboratory testified that he started documenting the blood trail, leaving evidence placards with letters and numbers along the way. There were so many stains he ran out of placards and had to use numbered orange cones to mark some stains, he said.

At first, in the dark early hours of Oct. 13, Zaveski said he thought the blood trail ended one block north of Lynch Street, at the corner of Varney Avenue and East 13th Street. But as he and officers looked more and after the sun came up, he said it was clear the trail continued to Leyden Street.

The last of the drops was in front of the Saalfield home. Rosales lived just a few doors away, although the two apparently didn’t know each other.

Zaveski said the blood stains appeared to be drips from a single source. “It seemed to be one continuous trail,” he said during questioning by Pearl.

Most of the blood stains — more than 30 of them — were on Lynch Street, Zaveski said. They became more spread out as the trail headed north, he said. It took 13 1⁄2 hours to document all the blood stains and to collect 28 of them for later DNA testing, he said.