Federal agents descended early Monday on Oaksterdam University — California's first cannabis industry training school — securing the downtown Oakland facility as they served search warrants.

A spokeswoman with the Internal Revenue Service said the raid is part of a joint investigation with the Drug Enforcement Agency but she declined to elaborate.

"It's an ongoing investigation and everything is under seal, so we're not able to comment," she said.

PHOTOS: Feds raid medical cannabis school in Oakland

Oakland in many ways has served as the cradle of California's medical cannabis movement and Oaksterdam University owner Richard Lee has been at the forefront. He runs a school, a plant nursery and a dispensary, and spearheaded California's recent unsuccessful ballot initiative to legalize marijuana.

The raid comes as federal officials are stepping up a crackdown on medical cannabis dispensaries and the landlords who provide them space in which to operate.