Of the dozens of bloodstains that came from the killing of a Huntington Station teenager, 11 of them seemed to present a problem for the man on trial in Riverhead, accused of her murder.

The jury hearing the case against Adam Saalfield, 22, had already heard about the trail of blood that led from the body of Maggie Rosales, 18, to near the Saalfield home a quarter of a mile away on Oct. 12, 2014. And jurors had heard about a bloodstained boot recovered during a police search of Saalfield’s bedroom 17 days later.

On Monday, forensic scientist Joseph Galdi of the Suffolk Crime Laboratory told jurors and state Supreme Court Justice John Collins whose DNA was in that blood.

Galdi’s colleagues swabbed 28 of the 59 groups of bloodstains they found on Lynch Street, where Rosales was stabbed twice in the neck as she walked toward Depot Road, and on Varney Avenue. Galdi said he tested 13 of the stains for DNA.

Most of them, particularly the stains near where Rosales was found in a pool of blood that was nearly the length of her body, contained DNA only from Rosales, Galdi said during questioning by Assistant District Attorney Rafael Pearl.

But stain 17, stain 19, stain 21, stain 22, stain 24 and stain 28 all told a different story. Galdi said these stains contained a mixture of DNA from both Rosales and Saalfield. The last stain, number 28, was found in the intersection of Varney Avenue and Leyden Street, where the Saalfield home is located.

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The probability that anyone unrelated to Saalfield contributed DNA to that bloodstain is 1 in 2.45 quintillion, Galdi said. The chance that the other DNA came from someone not related to Rosales is 1 in 19.4 billion, he said. There are about 7.3 billion people on the planet.

Galdi said there were five bloodstains on the right boot from the pair taken from Saalfield’s bedroom closet. The DNA in all five stains matched Rosales, he said. The probability that it could come from someone unrelated to her is 1 in 5.49 quintillion, he said.

Earlier Monday, Douglas Arevalo, 21, testified that he and Rosales dated “on and off” for five years. The night before she was killed, Arevalo said they smoked marijuana with a friend in a park in Huntington Station before sleeping together.

“She gave me a hug and gave me a kiss and said, ‘Take care,’ ” when he dropped her off at her home in the morning, Arevalo said.

Toward the end of his shift at a restaurant in St. James, he said, he tried texting and calling her, but got no response. He went to the park where they often got high together, but she was not there, he said.

The next morning, he said he went to 7-Eleven and got her cotton candy and some Skittles. “She liked those,” he said. But he went back to the park and still couldn’t find her.

Detectives came to his house later. At first he was worried it was about traffic tickets he hadn’t paid, but they took him to the Second Precinct nearby and asked him about Rosales.

“I thought it was odd,” Arevalo said, but then detectives told him she was dead. “I completely lost it.”

During cross-examination by defense attorney Craig McElwee of Hauppauge, Arevalo said the couple had argued the night before she was killed, but they had “talked everything out, and everything was OK.”

Arevalo said he was aware that a couple of Rosales’ previous boyfriends had been physically abusive to her.