Nassau County police on Thursday urged the public to be careful as they investigate a series of robberies since March involving a knife-wielding, masked man.

Officials also doubled the Crime Stoppers reward — from $5,000 to $10,000 — for information leading to the arrest of the man, who police believe robbed his 10th business at about 8:30 p.m. Wednesday, hitting a Carvel ice cream store at 1344 Peninsula Blvd. in Hewlett.

Police said the suspect has brandished a 6-inch knife during the evening and early morning robberies, which have targeted some Dunkin’ Donuts and Carvel stores.

Police released a sketch of the suspect and a video of what is believed to be his first robbery, on March 1, of a Dunkin’ Donuts in New Hyde Park.

The suspect entered the Hewlett Carvel before closing time on Wednesday night, went behind the counter and ordered an employee to open the cash register. He took cash and fled on foot, police said.

Two employees in the store at the time were not injured, police said.

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Police said the suspect is about 6 feet tall and wears a hooded jacket, scarf or bandanna across his lower face, and gloves and sunglasses.

Det. Lt. Richard LeBrun, a police spokesman, asked Nassau residents, whom he called “our greatest resource,” to help in the investigation.

He urged residents to “take a moment, look around” before they enter businesses at all hours. “If you’re on your cellphone, if you’re texting — put it down, put it away. Be cognizant of your surroundings.”

LeBrun said police patrols in the area of the robberies have been increased.

Police are also looking for a second knife-wielding suspect believed to have robbed the same Williston Park Dunkin’ Donuts shop twice.

On Wednesday at about 8:30 p.m., a heavyset man wearing a mask showed a knife and demanded an employee open the register of the Dunkin’ Donuts at 276 Hillside Ave. He left with an unspecified amount of cash and no one was injured, police said.

Police said the man may have also robbed the store on April 8 around 10:30 p.m.

Anyone with information about the robberies is asked to call Crime Stoppers at 1-800-244-8477. Information will be kept confidential.