The FBI has joined the hunt for a gas-station robber who has struck 15 times on Long Island since August -- a spree that has instilled fear in station clerks and motorists alike, officials said at a news briefing Friday.

Nassau police said the FBI was brought in to assist on the case because of the department's close relationship with the agency, and because the robber potentially faces federal firearms charges stemming from the heists.

The casually dressed, calm-looking suspect is seen on surveillance videos unhurriedly entering businesses throughout Nassau and Suffolk counties, always clutching a black, semiautomatic handgun covered by a cloth.

In each heist, the robber calmly walks toward the register, displays his gun and demands cash from a store employee. There are no customers present in the videos, leading detectives to speculate that the robber cased each station before entering.

"He's very cool, he moves very slowly," Det. Lt. Kenneth Catalani, commanding officer of Nassau's robbery squad, told reporters at police headquarters in Mineola.

The robber tells his victims to "give me the money, give me the money fast," according to police.

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"He's very demanding, and they move quickly," Catalini said.

In addition to publicizing new details of the crimes Friday, authorities released several pictures and videos taken during the robberies, and announced a $15,000 reward for information leading to an arrest.

Police in both counties also said they are increasing patrols of gas stations.

"These pictures that we have now are really probably the best evidence we have to solve this quickly," said Suffolk police Deputy Insp. William Doherty, whose agency is working with both Nassau police and the FBI to catch the robber.

The unidentified man has targeted gas stations across a 60-mile span, from Elmont to Riverhead, always leaving detectives one step behind. There is no geographic pattern to his crimes, police said, and investigators have not been able to determine whether he uses a vehicle.

"The breadth of the area is a dynamic that makes it more difficult," acting Nassau Police Commissioner Thomas Krumpter said.

Nassau police said the series of robberies began with a BP station holdup Aug. 4 in Hempstead, while the latest robbery occurred Wednesday at 10:38 p.m. at a Mobil station on Old Country Road in Westbury -- the ninth heist thought to be carried out by the robber in Nassau.

In Suffolk, police believe the same man also is responsible for at least six armed gas-station robberies.

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Acting Nassau District Attorney Madeline Singas said the crimes were particularly disturbing because of how dependent residents are on automobile travel -- and the workers who fuel their cars.

"It's scary, of course," Raji Singh, an attendant at a BP station on Jericho Turnpike in Mineola, said Friday. "They need to catch this guy." Police have not said how much money was stolen in the Nassau robberies, but Suffolk police said the suspect has taken between $500 and $850 in each of the heists there.

Anyone with information about the suspect is asked to call Nassau Crime Stoppers at 800-244-TIPS. Suffolk police also have asked anyone with information to call Suffolk Crime Stoppers at 800-220-TIPS.

All calls will remain confidential, police said.

With William Murphy