The above image was taken from a camera at the Chase Bank on Woodbury Road in Plainview on Tuesday afternoon.
It shows the suspect handing a teller a note, which allegedly demanded cash.
Police say he left empty handed.
They believe the same suspect robbed a TD Bank in South Huntington 40 minutes later.
Witnesses described the suspect as a 6-foot tall man wearing blue jeans, sunglasses and a white Nike baseball cap.
Detectives are requesting anyone with information regarding this crime to contact the Nassau County Crime Stoppers at 1-800-244-TIPS. All calls are confidential and all callers will remain anonymous.