Police in northern Mexico have raided a lab producing the synthetic opioid fentanyl, the second such lab detected in Mexico in the last four months.

Police in the northern state of Sinaloa said Thursday that the lab was discovered in a house in a middle-class neighborhood of the state capital, Culiacan.

The state's assistant public safety secretary says police were patrolling in an armored truck when they came under fire. Carlos Hernandez Leyva says officers chased the assailants into the house, but they escaped through a back door.

In December, Mexican federal agents arrested four people in a raid on a fentanyl lab in Mexico City.

Much U.S. fentanyl originates in China, but is often smuggled through Mexico. Mexican cartels also produce the drug from precursor chemicals imported from China.