The discovery of the mutilated bodies of four young men in a Central Islip park prompted Suffolk County’s top cop Thursday to declare war on MS-13, a violent street gang believed to behind a spate of slayings in the Brentwood area.

Meanwhile, a graphic video purportedly depicting the dead bodies was sent Thursday to the girlfriend of a missing teenager; his family said the video showed their son to be among the bodies.

Suffolk police did not immediately comment on the video. But in a news conference Thursday afternoon, Suffolk Police Commissioner Timothy Sini called the slayings “a stark reminder that we are in the midst of a war.”

“We’re going to continue that war,” Sini said, standing across the street from where the four bodies were found Wednesday night.

The victims — one age 16, two age 18 and the other 20 — had “significant trauma” to their bodies,” Sini said.

Police discovered the victims about 8 p.m. Wednesday, several hundred feet south of a soccer field in a park bounded by Clayton Street and Lowell Avenue. But Sini would not say how police were alerted to the crime scene.

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The four were apparently killed with a “sharp or edged instrument,” he said. While the commissioner said investigators “have not ruled out other possibilities at this point,” he added that the manner of the slayings was “consistent with the modus operandi of MS-13.”

Sources familiar with the investigation said the bodies were mutilated, worked over with both blunt and sharpened weapons, presumably bats and machetes.

Sini said the exact date and time of the killings won’t be released publicly, but said the four homicides occurred at the same time “within the past few days” at the park.

Police have tentatively identified the victims, but did not release the names Thursday.

But the parents of Justin Llivicura, 16, of East Patchogue, said their son was one of the four killed.

Blanca Zhicat and Marcelo Llivicura said their son’s girlfriend was sent a video of the bodies and in it they recognized their son, who was wearing the same clothes as when they last saw him Tuesday night. He had said he was going to a party in Manhattan.

When the girlfriend got the video Thursday afternoon, she alerted the parents and they said they called the police.

Llivicura was reported as a missing person to Suffolk police on Wednesday at 7:08 p.m., and the investigation began “within three minutes,” police said.

Relatives of Jorge Tigre, 18, said police told them he is among the dead. Tigre’s brother, William Tigre, 21, of Bellport, told reporters that he got a call from a friend at 7:30 p.m. Wednesday saying his brother was injured and at the park.

“He told me my brother was hurt. . . . He just told me, ‘quick your brother was there. I saw him there, and you’ve got to go there,’” William Tigre said.

Tigre said he spoke to investigators at the scene, but he did not get any information from them.

Jorge Tigre, of Bellport, had been reported missing to Suffolk police at 3:28 p.m. Wednesday and police began searching for him at 3:36 p.m., police said Thursday.

Police would not say Thursday whether the two young men were among the homicide victims.

Sini announced a $25,000 “fast cash reward” — to be paid within 72 hours and in conjunction with Suffolk Crime Stoppers — for information leading to the arrest of whoever committed the killings, and stressed that the department is working closely with the FBI to solve the case.

The discovery comes six weeks after more than a half-dozen members of MS-13 — the violent street gang with ties to El Salvador — were charged in the killings of Brentwood teens Kayla Cuevas, 16, and Nisa Mickens, 15, and an MS-13 member, Jose Peña-Hernandez, 18, officials said.

Sources said MS-13 members are under intense pressure under the gang’s violent anti-social code to “put in work” — that is, to kill those perceived as rivals, such as members of other gangs or people believed to have disrespected the gang. Refusal to “put in work,” according to the sources, can result in the reluctant gang member being targeted for death for defying the gang code.

And further increasing the pressure for violence, there are large numbers of gang members newly arrived on Long Island, mainly from El Salvador, who have yet to prove themselves unless they engage in violence, the sources said.

The killings of the Brentwood girls, who were found beaten with bats and machetes, and the discovery of several sets of skeletal remains of other young victims, shocked the community and resulted in Suffolk police conducting a monthslong sweep, resulting in more than 125 arrests on state charges, aimed at wiping out the street gang and gaining intelligence.

Sini said Thursday that investigators would continue with strategies, including concentrating on intelligence, that have been working to tamp down gang activity in an “all-hands-on-deck” assault. He added cops also would deploy new strategies that would not be publicly revealed.

“No doubt, we’ve made tremendous progress, we’ve solved several murders, including Nisa and Kayla. We’ve made hundreds of arrests,” Sini said. “But this is a long-term war. And make no mistake about it, it’s a war.”

With Sarah Armaghan and Mark Morales