Suffolk County Police haven't announced any arrests in the mysterious slayings, which have the community on edge.
Police Commissioner Timothy Sini said Monday there is an indication of "gang involvement" in all four deaths, but he declined to elaborate.
Nisa Mickens, 15; Kayla Cuevas, 16; Oscar Acosta, 19, and Miguel Garcia-Moran, 15, had attended the same high school in Brentwood, a working-class community about 45 miles east of New York City. Mickens and Cuevas were beaten to death, CNN affiliate News 12 Long Island reported. Acosta and Garcia-Moran also were victims of assault, according to Sini.
The first sign of tragedy came when a passerby found Mickens' body on a Brentwood street on September 13, the eve of her 16th birthday, according to Sini. The following day, the body of 16-year-old Cuevas, who was Mickens' best friend, was discovered in the backyard of a nearby home after the two had gone for a walk the night before, News 12 reported.
Sini said the two girls were likely together when they were attacked. Authorities have offered a $15,000 reward for information leading to an arrest in the two slayings.
"The murders of Nisa and Kayla are related," said Sini, who added the two girls died of blunt-force trauma. "For any community to have girls brutally murdered on the streets is appalling. I know the Brentwood community is not going to stand for that."
Two more bodies
Nine days later, Acosta's and Garcia-Moran's remains were found in an industrial and wooded area of Brentwood. Their deaths are being probed as independent investigations, Sini told CNN.
Garcia-Moran was reported missing in February and Acosta was reported missing in May, police said. Sini wouldn't say what led police to their bodies on September 23 or whether they had been buried.
Garcia-Moran's stepfather said his son disappeared when he went to meet friends at Brentwood High School, News 12 reported. The teenager's father said he feared the boy may have been involved with a gang, News 12 reported
Sini wouldn't say if any particular gang was involved. He said authorities are working to solve the crimes and to ensure residents' safety in several ways, including increasing police presence and employing specially trained officers to crack down on gangs.
"The only people who have anything to fear are the criminals," Sini said.
But the killings have unnerved some parents in this ethnically diverse community.
"After they found those two girls, we don't even let my daughter walk (outside) anymore," Fidel Cabrera, a parent, told News 12
'Fighting with love'
At Mickens' funeral, a pastor eulogized her by praising her bravery, telling mourners it appeared she had tried to fight off the attack.
"I said, 'We're not just burying a young girl, we're burying a hero because she went out fighting,'" said the Rev. Bryan Greaves, 27, associate pastor of Holy Church of Christ in Central Islip, New York, in an interview with CNN.
Sini couldn't confirm that Mickens put up a fight during the struggle. She had suffered at least 10 injuries to her head or face, he said.
Mickens and Cuevas were juniors at Brentwood High, while Garcia-Moran was a sophomore and Acosta was not a student at the time of his death, school officials said.
The Brentwood Union Free School District has since taken security measures, such as reinstating metal detectors at Brentwood High School and increasing police presence, said district spokesman Felix Adeyeye.
Suffolk police and community members also have held a forum to address concerns about safety. And Greaves, the pastor, organized a community peace and prayer march.
Greaves said he challenged mourners at Mickens' funeral to start "fighting with love, fighting with peace."
Greaves told CNN the community is still vibrant -- much like Brentwood's school colors, which are green and white -- and will recover.
"Brentwood had a tragic moment, but the grass is still green," he said in an interview. "The pride is not gone. We're still who we say we are."