Suffolk police have found the skeletal remains of a Brentwood teenager — a known MS-13 gang member — who police believe was beaten to death by gang members, police said Thursday.

The discovery of the remains of Jose Peña-Hernandez, 18 — unearthed by Suffolk police on Monday in a wooded area on the grounds of the Pilgrim Psychiatric Center — marks the sixth gang-related homicide by assault in Brentwood discovered by police in the last month.

Suffolk County Police Commissioner Timothy Sini said Thursday that investigators have rounded up more than 30 MS-13 gang members since the gang-related beating deaths of two teenage girls last month, in what Sini has described as a mission to “decimate” the violent gang that has terrorized the community.

Sini said police, in conjuction with federal authorities, have put five “violent” gang members in federal custody, and another 25 gang members have been arrested on state charges since Sept. 13. A sixth gang member was taken into federal custody soon after the slayings.

Peña-Hernandez, who according to school district officials dropped out of Brentwood High School’s Sonderling Center in 2014, was a known MS-13 gang member who was reported missing June 13, police said.

Peña-Hernandez is one of three teenage boys who were reported missing this year and whose skeletal remains were found in the last month near the psychiatric center in an apparent MS-13 dumping ground. Police believe all three homicides were likely perpetrated by gang members, but wouldn’t say whether they believe the killings were committed by the same person or persons.

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The teen boys’ remains were found following the gang-related killings of Brentwood High School students Nisa Mickens, 15, and Kayla Cuevas, 16 — best friends whose deaths sparked both community outrage and an aggressive police response.

The commissioner would not identify the alleged gang members or specify whether the charges are related to the six deaths, except to say that those in federal custody were facing RICO charges, a federal statute created to prosecute the mafia that in recent years has been used to target street gangs.

Sini, who before becoming police commissioner was a federal prosecutor, called the statute “a good tool to dismantle criminal enterprises.”

The commissioner also declined to directly link any intelligence that may have been culled from the 31 arrests to the discovery of the bodies of the three teen boys, but said, “you don’t just stumble upon skeletal remains.”

Sini said breaks in the cases were “a result of additional intelligence that we’re collecting as this investigation progresses.”

“We didn’t find him by accident,” Sini said. “We didn’t find those remains by accident. We are putting an enormous amount of pressure on the criminal element in the Brentwood area.”

In the days after the killings of Cuevas and Mickens, Suffolk police investigators began conducting grid searches with K-9’s and shovels on the grounds of Pilgrim and in an industrial park area nearby.

When the teenage boys went missing, police believed early on in the investigations that “foul play was involved,” Sini said, and Chief of Detectives Gerard Gigante transferred the missing persons cases to the Major Case Unit for investigation. Sini declined to specify what indicated foul play to investigators. One of the six gang-related killings involved an adult, Dewann A.S. Stacks, 34, who was beaten to death on Oct. 13 where he had been walking along American Boulevard, police said.

Family members of Peña-Hernandez could not be reached for comment.

“This is very devastating news on our community here to trying to heal from the other murders we currently have,” superintendent Levi McIntyre said in an interview. “This again puts us in a very difficult situation, particularly to know he was a former student of ours. We just have to work with our community in order to support each other and try to make our schools safe.”

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He added that security remains at a “heightened” level. “They should not be afraid to come to school because our schools are safe,” McIntyre said.

Citing added security measures brought in September after the discovery of the first bodies, McIntyre said, “things are quiet and calm in our schools.”

— With Scott Eidler