Police found slightly more than 33 pounds of "China White" while searching a taxi passenger for gun Monday in Methuen.
The street value of the drugs is estimated at $1.2 million.
Police were called to Glen Avenue that morning when a witness called 911 to report a man with gun, WCVB reported. The caller reportedly gave a detailed description of a man and what he was wearing.
The officers located, later identified as Robinson Rojas-Rosario, 32, of New York City, according to the Boston Herald.
When officers patted him down, they didn't find a gun, they found 15 packages marked "China White." Methuen police said it is a blend of opioids that usually includes fentanyl.
Rojas-Rosario, who lives in Queens, faces drug trafficking charges, police said.