Police are trying to identify the man they say robbed 21 stores, gas stations and restaurants in Nassau County over the last 11 months.

The robberies have been in Lynbrook, Franklin Square, Elmont, Hewlett and, most recently, a Valero gas station in Valley Stream Sunday.

Detectives say the suspect threatens victims with a large knife and demands cash. He's also been stealing cigarettes.

Police say 15 of the 21 robberies have occurred in the Valley Stream area.

Police recommend store owners keep their stores brightly lit and their windows clear so people passing by can see inside. They also say merchants should comply with orders given by the robbery suspect.

Authorities say the robber mostly likely lives in the Nassau area and cases each target before he strikes.

Anyone with information is asked to call Nassau Crime Stoppers at 1-800-244-TIPS. There is a $5,000 reward offered for information leading to an arrest.