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

A spokeswoman with the Internal Revenue Service said the raid was 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.

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 the school, a plant nursery and a dispensary, and he 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.

Despite a 1996 California law that legalized marijuana for medicinal use, the federal government has maintained that the dispensaries still violate federal law.

Oakland officials have expressed continued support for well-regulated dispensaries. Just three weeks ago, Oakland city officials issued preliminary approvals for four new medical marijuana dispensaries, which would bring the total to eight.

In addition to the raid on Oaksterdam, federal agents raided Lee's home Monday. He was detained for questioning and released, said Drug Enforcement Administration special agent Joycelyn Barnes.

After the raids began Monday, Lee supporters rallied outside Oaksterdam, where federal agents stood behind yellow police tape.