New York City authorities have seized nearly nine pounds (5 kilograms) of fentanyl with a street value of $10 million hidden inside fish.

The synthetic opioid was recovered last week from a vehicle in the Bronx. The brick-shaped packages were wrapped in fish and placed inside Styrofoam coolers.

Police said Thursday that Johnny De Los Santos-Martinez, of the Bronx, was arrested on drug possession charges in connection with the fentanyl seizure. Officials say narcotics detectives had him under surveillance. Information on his lawyer wasn't immediately available.

Fentanyl is up to 50 times as strong as heroin and is frequently mixed with heroin, cocaine and other synthetic drugs.