The body of Mickens, 15, was spotted by a passing motorist on Stahley Street near Loretta Park Elementary School in Brentwood at about 8:30 p.m. on Tuesday.
Cuevas' body was discovered Wednesday evening in a wooded area behind a house, only a few hundred feet from where her best friend's body was found.
"We need your help. This is an act of savagery in our community and we need to work together to bring the perpetrators to justice," Suffolk County Police Commissioner Timothy D. Sini said.
No one knows where Cuevas and Mickens, who attended Ross High School, were heading when they left Cuevas' house in Brentwood sometime after 6:30 Tuesday night.
Both families believe the girls were walking back to the house when they were attacked.
There are strong indications that these murders were committed by members of a gang, Sini said. They are making a list of known gang members in the area and working with federal law enforcement to solve these cases.
It's not clear why the girls were targeted.
Mickens had more than 10 wounds to her face and head, according to the medical examiner. Both died from blunt force trauma, Sini said. They also had wounds consistent with cutting.
"These murders show a level of brutality that is hard to match. Some of the worst that I've ever seen," Sini said.
Wednesday would have been Mickens' 16th birthday. She was an 11th grader at Ross High School and was on the basketball team. She was wearing her basketball jersey Tuesday night.
"I find this really horrible. These are kids. They are 16 years old. Everyone's lives should matter. Two little girls walking down the street," resident Erin Spano said.
A $5,000 reward is being offered for information leading to an arrest.
"When you murder two young girls who attend the local high school, the community is going to act," Sini said.
Authorities have stepped up patrols in the area, particularly around the high school. Metal detectors and hand wands were used to check for weapons.
Some students say all of the measures not enough to make everyone feel safe.
"It was pretty empty. A lot of kids stayed home I mean due to the fact that they were just scared themselves, but I feel pretty safe. I feel like the security here was pretty good," Luangelice Paraboo said.
"Everyone is like quiet. Like sad mood. Like one of my teachers I went up to her, she's usually really happy and I gave her a hug and tears were falling from her eyes. It was pretty sad," Pedro Garcia said.
Anyone with information that could assist investigators in this case is urged to call 1-800-220-TIPS. You can make tips anonymously.
A candlelight vigil will be held Friday night at 6:30 p.m. at Ross Memorial Park. Community leaders will walk from the high school to the park. They are doing this to take back their streets.