One of two teenage girls killed in a beating attack in Brentwood 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.”

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.”

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Cuevas’ body was found in the 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. 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.

Her mother, Evelyn Cuevas, said she didn’t want to 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.”

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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.”

Sini said a tip from a community member led to the discovery of Cuevas’ body.

Asked about the delay in finding Cuevas’ body, Sini said: “If you walked into the backyard of this property, you could not see the body because it was against two sides of a fence, and was behind a depression, or I should say it was behind an incline in the yard.”

He added: “Obviously, if the body was there when we were searching, we should have found it. It wasn’t because of lack of effort. These things are challenging. We used k-9, we used aviation.”

The day after the students were found beaten to death, Suffolk police increased their presence at the school, Brentwood High School’s Ross Center, officials said. Sini said threats were made on social media against the school, and police investigated but there is no current threat to student safety. He would not say if any arrests were made.

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Juliana Alfaro, whose daughter is a 10th grader at Brentwood High School, said she kept her daughter home from school Thursday because she was afraid of an outbreak of violence. She visited the scene of the slayings Thursday and hugged the grandfather of Nisa Mickens and told him that the school’s parents are praying for his family.

“I’m afraid to send her, because what about they are getting out from the school and one crazy gang is shooting outside. You never know. My daughter is just 15 years old, you know? How’s everyone going to protect them. I’m really nervous. “

School officials sought to allay fears.

“Please know that we are taking active steps to ensure the safety and welfare of our students,” Schools Superintendent Levi McIntyre said Thursday in a letter to parents.

“We have reintroduced the use of the metal detectors along with the handheld wands at the high school to ensure that no weapons are brought into our buildings,” he said.

McIntyre said the school’s psychologist, social worker and guidance counselors were providing grief counseling.

In a statement Suffolk County Executive Steve Bellone extended “our most heartfelt condolences” to the families of the victims. “Words cannot express the sorrow we feel for the senseless loss of Nisa Mickens and Kayla Cuevas,” Bellone said. “These were two extraordinary young ladies with their whole future ahead them.”

Freddy Cuevas, Kayla’s father, came Thursday afternoon to the scene of the slayings, where a growing memorial of balloons, flowers and other mementos has formed.

He knelt in the street and wailed as he mourned his daughter.

“Kayla was the best, the best,” he said, as he walked back to his home about a block away. “She was an athlete. She was the best. She was incredible. She was something else. She was an amazing star.”

All day and into the night, family and friends of the girls -- as well as strangers -- stopped by to pay their respects.

Concetta Imperato, 56, a Brentwood resident, brought flowers, even though she didn’t know the girls.

“It’s horrible, just very sad,” she said. “It’s been a rough town for many years. I’m hoping it will strengthen it, bring the people together, more aware of what’s going on with the gangs and everything.”

Lenny Tucker, president of the Brentwood Association for Concerned Citizens, arrived at the Ross Center Thursday morning shortly after school opened. He said he anticipated “a lot of emotional turmoil” at the school.

He said there’s a “spectacular” group of students who are monitoring social media to see if there is any information they can share.

“It’s a sad situation,” he said. “We’ve been fighting to quell gang violence for the last 10 years in Brentwood. ... And it seems to be starting to raise its ugly head again.”