An anonymous private $5,000 donation has increased the reward to $10,000 for information leading to an arrest in the murder of two teenage girls in Brentwood, police said Friday.

One of the girls killed in the beating attack had a dispute in school with students believed to be members of the MS-13 street gang, and police are investigating whether it played a role in the killings, a law-enforcement source said Thursday.

Suffolk County Police Commissioner Timothy Sini said Thursday it’s likely the slayings were committed by gang members and that both girls died of blunt force trauma.

Sini, a former federal prosecutor before he was appointed commissioner, said at a news conference that the girls’ injuries “were some of the worst wounds I’ve ever seen.”

He declined to say whether MS-13 was the gang involved with the killings or whether Kayla Cuevas, 16, had a confrontation with members of MS-13, the street gang with ties to El Salvador.

But a source with knowledge of the investigation said Cuevas “had a disagreement with MS-13.”

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The source added: “That’s what we’re looking at right now, just to see if there’s any nexus there. She had a dispute with some kids in school and there was a fight last week and it involved some gang comments and some signs.”

Cuevas’ body was found in a wooded area of a home on Ray Court in Brentwood Wednesday evening — less than 24 hours after the body of her friend Nisa Mickens, 15, was discovered by a passing motorist Tuesday night on Stahley Street at Ray Court. The two girls had grown up together.

“There are indications these murders were committed by members of a gang,” Sini said at the briefing Thursday at the Brentwood Fire Department. “Both the girls “suffered significant injuries to the head and face area among other injuries. They both died from blunt force trauma. They both had cut injuries as well,” he said.

The commissioner pledged “to get to the bottom of this brutal, heinous double homicide.”

“The people who committed this crime, the person or people who committed this crime, are severely depraved to cause injuries of this nature,” Sini said. “You have to have no regard for human life. . . . These murders show a level of brutality that is close to unmatched.”

There’s no indication that Nisa, who was killed the night before her Sweet 16, was involved in the school fight with the MS-13 gang members, the source said, adding that investigators believe there are likely two or more suspects but are still in the early stages of their probe.

“When you deal with gangs, if they feel disrespected, it could be anyone in that gang,” the source said. “That’s how gangs work.”

The families of both girls sobbed at the scene Thursday, as they visited a growing sidewalk memorial of balloons, flowers and cards left for the popular teens.

Evelyn Cuevas, the mother of Kayla Cuevas, wouldn’t discuss a possible motive or the involvement of gangs, but said through tears: “She was trying to keep focused, but this nonsense that is out here, it’s hard for kids. It really is. They get bullied if they don’t participate, They beat them up, they kill them. It’s not right, taking another’s person’s life. I gave birth to her. I just want justice for both of them.”

Sini declined to specify whether investigators were close to making an arrest, but he asked anyone with information to contact police. He said Suffolk CrimeStoppers is offering a $5,000 reward for help in solving the case and urged members of the community to call “regardless of criminal history or immigration status.”

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That reward was increased to $10,000 with the anonymous $5,000 donation, police said.