Nassau police said Monday they suspect a lone, masked gunman has made off with at least $13,000 in holdups of 14 retail stores across Long Island since August.

The suspect often has used zip ties or handcuffs to bind customers and workers and forced employees to open safes or cash registers, police said.

The latest robbery happened early Monday morning when the masked gunman forced his way inside a Westbury Babies R Us after an employee opened a door to let a co-worker in, police said.

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One female employee was handcuffed and told to lie on the floor before the gunman forced another employee to put money from a backroom safe into a bag, handcuffed her and fled, police said.

Monday’s holdup added to a tally that now includes eight stores in Nassau and six in Suffolk since Aug. 20, police said. In each robbery, witnesses described the suspect as a man clad in a black mask, dark clothes and carrying a black handgun.

No one has been injured in any of the robberies, but police are urging both the public and retail employees to be on alert — and to comply with any robbers’ demands.

“We’re extremely concerned about this robbery pattern,” said Det. Lt. Richard Lebrun, commanding officer of Nassau’s Public Information Office. “We have dedicated a large amount of resources to patrolling in areas that he might hit again. . . . Hopefully we get some tips and no one gets hurt.”

Police in Nassau said they have intensified the frequency of patrols around retail stores, including plainclothes and other special unit officers.

The thief has gotten away with between $800 and $5,200 during each robbery in Nassau, police said.

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Suffolk police declined to say how much the robber has netted. Earlier this month, they said patrols would be increased near businesses after the suspected robber held up a Beach Bum tanning salon on Jericho Turnpike in Huntington Station.

Suffolk police said then that they suspected the gunman — linked by Nassau police Monday to several Nassau robberies — tied up two employees and two customers on Nov. 22 inside a Pier 1 Imports in Huntington Station before fleeing with cash.

Anyone with information can call Nassau County Crime Stoppers anonymously at 800-244-8477.

— With Patricia Kitchen