Two co-eds from Long Island — best friends since high school — were gunned down in Arizona Monday by a suicidal man who was furious at one of them for convincing his girlfriend to dump him, police said.

Carol Kestenbaum, of Bellmore, an Arizona State University student, had been celebrating her 20th birthday with her pal Nicole Schiffman, of Merrick, who had flown in from Maryland, where she attended school.

After the celebration, the young women got into Kestenbaum's Mercedes and drove to her off-campus apartment complex at 4 a.m., Tempe Sgt. Dan Masters said.

The suspected killer, Joshua Mendel, 22, was waiting for them. The complex is gated but does not have guards and the gunman likely slipped in behind another resident.

Mendel first shot Kestenbaum — a friend and former roommate of his ex — in the head. Then he turned the gun on Schiffman and fired several shots into her back, police said.

Schiffman, 19, "appeared to be running away," Masters said.

Mendel then shot himself in the head. His body was found about 15 feet away from Kestenbaum. Two guns were recovered nearby.

"It was a very bloody scene," Masters said.

"We believe he came over here to confront her about whatever she didn't like about him and the relationship."

Kestenbaum's mother, Rita, said her daughter was "trying to help [the roommate].

"She told her, 'Why are you going with him? I know you can do better.' "

Laurie Fontana, a friend of Kestenbaum, said the victim "had a really bad vibe about him [the killer]. She found him creepy and told her roommate to break up with him — and she paid for it."

Masters said Mendel's unidentified ex was "not hurt" but was "distraught."

Schiffman did not know her killer and "was in the wrong place at the wrong time," Masters said.

Schiffman's grieving father, Ronald, said she was studying journalism at the University of Maryland and described her as a "very creative, lovable person."

He said he'd remember her "as a person who was interested in the world and [who] couldn't understand why there is so much violence."

Kestenbaum had recently decided to become an education major.

"She was loved by everybody," said her mother. "She was the light of my life."

The mother said the victims, who had attended John F. Kennedy High School in Bellmore together, had spent their final day on earth "laying in the sun."

One of Kestenbaum's friends, Honora Swanson Bober, told The Arizona Republic that she had gone with both victims to see the musical "Rent" at ASU on Saturday night.

The news shook Kestenbaum's close-knit Long Island neighborhood.

"My children are devastated," said neighbor Ileene Adler.

"She's a beautiful young woman, lovely girl, very loved in the neighborhood."

Additional reporting by Erin Calabrese and Tracy Cunningham