Police say a man suspected of more than a dozen knifepoint robberies across Long Island struck again for a third time this week, and the FBI is now assisting police in the case.

The latest robbery happened around 9:45 p.m. Wednesday at a Subway restaurant on Sunrise Highway in Lindenhurst. Police say the robbery fits the pattern because the robber was calm, waited until the store had no customers and he used a knife.

John Kremnitzer says he was on his way to work last night at a 7-Eleven a few doors away from the Subway while the robbery was happening. He believes that the robber knows that many stores instruct employees to give up money without a fight.

Kathy Tavormina, of Lindenhurst, says she worries about the safety of her daughter, who sometimes works nights at a business in Babylon.

Earlier this week, the man robbed a Dunkin' Donuts in Elmont on Tuesday and a 7-Eleven in Westbury on Wednesday, according to police.

Police say the robber has targeted 17 businesses in Nassau and Suffolk since early March. Those businesses include 7-Eleven, Carvel, Dunkin' Donuts and Subway stores.

Police say that catching the knifepoint robber is a priority for them, and Nassau County is offering a $10,000 reward.

Suffolk police announced Thursday that the FBI is assisting both them and Nassau police in the case.

"Essentially we're doing stepped-up patrols to prevent further robberies," says Suffolk Police Commissioner Timothy Sini. "Having the FBI on board is critical because...they can bring certain technical resources to the table more easily than sometimes the local police departments can."

Anyone with information on the suspect or robbery spree is asked to call Crime Stoppers at 1-800-220-TIPS.