Jesalyn Harper, the only full-time code enforcement officer for the small, agricultural city of Reedley in California’s Central Valley, was responding to a complaint about vehicles parked in the loading dock of a cold-storage warehouse when she noticed a foul smell and saw a garden hose snaking into the old building.

A woman in a lab coat answered her knock, and behind her were two others in plastic gloves and blue surgical masks, packing pregnancy tests for shipping. Harper said they spoke broken English and told her they were from China. Walking through the lab, she found dozens of refrigerators and ultralow-temperature freezers hooked to illegal wiring; vials of blood and jars of urine in shelves and plastic containers; and about 1,000 white lab mice being kept in crowded, soiled containers.

The women said the owner lived in China, provided a phone number and email address and asked her to leave. Alarmed by what she saw, Harper, whose work mostly entails ensuring people have permits for yard sales and are keeping their lawns mowed, contacted Fresno County health officials and then the FBI.