There were few outward signs of the thriving heroin stash house that federal officials say was being operated on a quiet block in the Bronx across from Van Cortlandt Park. No one seemed to be permanently living in the apartment, 5E, at 210 West 251st Street, just outside the exclusive Fieldston neighborhood, officials said.

But as federal agents raided the hide-out over the weekend, they upended what they described on Tuesday as a major heroin-importing organization that had ties to an area in Mexico controlled by the Sinaloa Cartel and that had been receiving multimillion-dollar shipments of the drug at least once a month since 2013.

Traffickers from Mexico even visited New York to see things for themselves, taking in the sights and visiting Times Square, officials said.

The seizure on Sunday capped an elaborate yearlong investigation, yielding 70 kilograms of heroin — or nearly 155 pounds — that investigators say came from Mexico and was sold in chunks to various dealers for distribution upstate and across the Northeast.