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From the outside, there was nothing especially notable about the small white building on the corner of a cobblestone street in TriBeCa. But until recently, it was a crucial location in a sprawling empire.

“Beautiful Loft Prime Tribeca 4BR/2BA Sleep 10,” read the listing on Airbnb for one apartment there.

Two of the three apartments in the building were popular with tourists looking to stay in one of Manhattan’s most desirable neighborhoods — at $600 a night each, they were a bargain for a large group.

But they were also illegal — part of an elaborate real estate scheme to make millions by circumventing state and local laws and Airbnb’s own rules.