California authorities struck a high point in their investigation after receiving a tip about allegedly legal hemp production that led them to fields of approximately 10 million illegally grown marijuana plants.

The Kern County Sheriff’s Office said in a statement that it executed search warrants, along with the California Department of Fish and Wildlife and the FBI, on 11 fields across 459 acres in the area of Arvin—which is 100 miles north of Los Angeles.

Authorities tested the plants, finding that the THC levels in the crops were “beyond the legal limit.” THC is the psychoactive substance within marijuana.

The Kern County Narcotics Unit promptly destroyed and seized the 10 million marijuana plants worth $1 billion on the black market.

The California Food and Agriculture Department Code states that industrial hemp must have less than 0.3 percent THC content.

“These illicit marijuana gardens were grown under the guise of legitimate hemp production,” the sheriff’s office said.

Authorities say the investigation is ongoing.