The FDNY uncovered a basement labyrinth filled with slot machines, bootleg merchandise and sketchy cubbies with mattresses underneath two Manhattan buildings that house a massage parlor, officials said.

FDNY Fire Protection Inspectors were investigating a complaint about unlawful charging of hazardous lithium ion batteries inside the connected buildings on Elizabeth Street near Grand Street around 12:30 p.m. Tuesday when they found the batteries — and lots of other illicit items, the department said.

An area of the basement was converted into a gambling den packed with 17 slot machines, and a storage space held an assortment of counterfeit designer bags and accessories, according to officials.

FDNY Fire Protection Inspectors were investigating a complaint about unlawful charging of

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