The accident happened on South Country Road at 9:30 Thursday night as the bicyclist identified as 22-year-old Eric Dennis tried to cross the road.
A Jeep traveling westbound hit him first. That driver, identified as by 56-year-old Deborah Lamm, stopped.
The impact threw Dennis into the eastbound lane.
He was struck a second time by a light-colored sport utility vehicle driving eastbound. This vehicle then continued driving east without stopping.
Dennis was taken to Stony Brook University Medical Center in serious condition. He later died.
Police impounded the 2000 Jeep for a safety check.
The investigation is continuing. Detectives are asking anyone with information to call the Fifth Squad at 631-854-8552 or anonymously to Crime Stoppers at 1-800-220-TIPS. All calls will be kept confidential.