Nearly 20 pounds of heroin and fentanyl was seized from a Central Park West apartment that was uncovered as a drug packaging mill last week, officials said Monday.

Four men were arrested following the seizure of the drugs, which have an estimated value of at least $3 million, prosecutors said. Inside the apartment in 448 Central Park West, across from the park on West 105th Street, investigators also found a loaded gun between two couch cushions, $30,000 in cash and several rubber branding stamps with names like “Uber,” “Black Friday,” “Panda” and “Walking Dead.”

“This seizure alone contains enough potency to kill half of the population of New York City, if laboratory analysis proves it is all fentanyl,” Drug Enforcement Administration Special Agent in Charge James J. Hunt said in a statement. “Fentanyl is the deadliest street drug to ever hit this country.”

Investigators were watching the building on Aug. 4 when they saw David Rodriguez leave with two boxes inside a white shopping bag, the New York special narcotics prosecutor’s office said. He got into the back seat of an Uber, driven by Richard Rodriguez, they said.

The investigators followed the car and stopped it at 121st Street and Amsterdam Avenue. They found large cylindrical packages containing a “tan powdery substance” inside the boxes, which they suspect was a combination of heroin and fentanyl, a synthetic opioid that is about 50 times stronger than heroin, prosecutors said.

Two other men, Jesus Perez-Cabral and Johnny Beltrez, exited 448 Central Park West later that day. Perex-Cabral was recognized as a member of the drug trafficking organization and when approached by authorities, he admitted to having a gun and drugs inside the apartment, prosecutors said.

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The investigators later found two large ziplock bags with the rest of the heroin and fentanyl inside the apartment.

The four men were arraigned in Manhattan Criminal Court on Saturday.