Authorities arrested five people and seized $4 million worth of heroin in a Bronx heroin mill bust Wednesday.

A task force made up of members of the DEA, the NYPD and New York State Police had been conducting surveillance on the apartment building on Nelson Avenue in Morris Heights, according to Special Narcotics Prosecutor Bridget Brennan. 

Investigators saw a man leave the apartment with a backpack and trailed him as he drove to the Cross County Mall in Yonkers. There, he met a couple in another car and slipped the backpack to them; when investigators pulled them over, they found one and a half kilograms of heroin in the backpack, said Brennan. 

A search warrant executed at the apartment turned up 16 kilograms of heroin, scales, grinders and other drug paraphernalia. Dozens of stamps used to brand the heroin had names like 7Up, HIV, Came to Kill and Crocodile, Brennan said. 

Three men in the apartment were arrested, along with the couple pulled over in their car. They were to be arraigned in Manhattan Criminal Court Thursday. 

Brennan also warned Thursday that black-market fentanyl is most commonly mixed into heroin, and users often don't know they're taking fentanyl. Fentanyl was a factor in nearly half of the 1,374 drug overdose deaths in New York City last year (an all-time high, and a 46-percent increase in overdose deaths from 2015, according to data from the NYC Department of Health and Hygiene).