Suffolk police found a second set of skeletal remains in Brentwood Wednesday and FBI agents and federal prosecutors believe an extremely violent group of MS-13 gang members may be responsible for those killings and the slayings of two teenaged girls last week, according to sources familiar with the investigation.

One of those gang members is being held on federal charges connected to an unrelated case of violence. But the sources said the gang member may be involved in one or more of the recent Brentwood killings, including the first set of remains found Friday. He has not been charged in connection with those deaths.

The arrest papers and the charges are under seal. The gang member, who was not identified, was arrested several days ago in a joint investigation between Suffolk police and federal law enforcement officials, the sources said.

The set of remains discovered in Brentwood on Wednesday afternoon is that of a 17-year-old youth who was killed, the sources said. No other information was known about that victim, according to the sources.

The first remains found Friday were identified Wednesday by police as Oscar Acosta, 19, who had gone missing May 2. Suffolk police said he was a homicide victim based on physical evidence but would not elaborate.

Acosta’s family declined to comment Wednesday.

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The discovery of Acosta and the other teens’ remains comes as the Brentwood community is already reeling from the gang-related killings of Nisa Mickens, 15, and Kayla Cuevas, 16, whose bodies were found badly beaten in Brentwood last week.

A Suffolk police source said detectives were looking at four cases of missing Brentwood teenagers this year, and when it was clear the cases had ties to MS-13, they were transferred to a multiagency gang task force. In at least two of the cases, each was last seen with someone who was either an MS-13 gang member or had some sort of association with the gang, the source said.

But it was not clear whether the remains found Wednesday and Friday were among those of the missing four.

At a news briefing at Suffolk Police headquarters in Yaphank earlier Wednesday, Police Commissioner Timothy Sini first referenced the possible suspect, saying there was a “violent known gang member in federal custody” who was “operating in the Brentwood area.”

But Sini, a former federal prosecutor who said police “pitched” the case to federal prosecutors and they “accepted the case for prosecution,” wouldn’t identify him or say whether he is being charged in connection with any of the killings.

Since the slayings of Mickens and Cuevas, Sini has stressed that investigators are cracking down on gang members in the community and promised federal prosecutions, citing the potentially longer prison sentences for those convicted in the federal system.

“We’re applying pressure to those individuals,” Sini said Wednesday. “The Suffolk Police Department stands with the Brentwood community. We’re going to do everything in our power to get to the bottom of the murders of Nisa and Kayla. ...And we’re going to do everything in our power to decimate gangs in the Brentwood community.”

The second set of remains, the 17-year-old youth, was discovered Wednesday in a wooded, industrial area — west of Emjay Boulevard, east of Sagtikos Parkway and north of the Long Island Rail Road tracks — the same area where Acosta’s remains were found Friday, which is a few miles from the location of the suspected gang killings of Mickens and Cuevas.

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Cuevas 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 has said.

Brentwood High School Principal Richard Loeschner said that Acosta was a student at Brentwood High School’s Ross Center at the time of his disappearance but did not have more details.

Loeschner said he had just attended services for the two girls earlier Wednesday. “It’s been a rough week. I feel terrible,” he said.

“The district is still shocked by the news of the two girls that we have to mourn this week, and this news is another shocker to the district. We are actively working with Suffolk police to provide any information that can assist with the investigation,” said Felix Adeyeye, a spokesman for the district, who said security has been heightened throughout the district, particularly at the high school.

Neighbors said the Acosta family has lived in the house for less than a year and one man who lives several houses away said news of Acosta’s murder has heightened fears of gang activity.

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The neighbor, who declined to be identified for fear of being targeted by gangs, said he and his wife, who have two young sons, started seriously talking about moving before the recent killings, but now the talks have intensified.

“You can only keep your child so protected at home, but then they have to navigate through a battlefield to get an education,” he said. “I’m not going to deal with that. I’m going to do what I can so they have better opportunities.”