By Dan Whitcomb

LOS ANGELES, June 6 (Reuters) - Federal authorities opened the latest front in their war on California's massive medical marijuana industry this week, filing property forfeiture lawsuits in a bid to shut down three dispensaries and sending warning letters to 34 people.

The moves by the U.S. Attorney's Office in Los Angeles are the latest in an ongoing crackdown on what federal prosecutors say is a flourishing network of illegal cannabis suppliers operating across California under the cover of the state's medical marijuana law.

Authorities have now targeted more than 220 dispensaries -- deemed "marijuana stores" by prosecutors -- or indoor cultivation houses in the seven-county Central District of California alone, U.S. Attorney's spokesman Thom Mrozek said.

Most have been closed or are facing eviction or have been the subject to additional federal law enforcement actions, he said. The Central District of California represents the largest federal law enforcement jurisdiction in the country.

"Because there are so many in our district we could not go after all of them at once, so we've developed a strategy based on geographical areas," Mrozek said. "We're methodically going through our district."

The forfeiture lawsuits were filed on Monday in U.S. District Court in Los Angeles against a pair of buildings in the Los Angeles suburb of Santa Fe Springs, one of which houses two dispensaries.

Letters were also sent to 34 other property owners or store operators in the Los Angeles area giving them 14 days to come into compliance with federal law or face civil or criminal actions.

The drive by federal prosecutors to shut down dispensaries has caused friction between the U.S. government and California, which in 1996 became the first state to decriminalize medical marijuana. Sixteen states and the District of Columbia have followed suit.

The possession or sale of marijuana is illegal under federal law, which does not have an exemption for medical purposes.

California Attorney General Kamala Harris, in a November interview with the New York Times, said the federal campaign had "only increased uncertainty about how Californians can legitimately comply with state law."

Harris also said federal authorities were "ill equipped to be the decision makers as to which providers are violating the law."

But Mrozek said federal prosecutors believed that the dispensaries were operating outside of California law, which he said permits only primary caregivers to dispense marijuana and bars sale of the drug for profit.

"Our position is that they are in violation of federal law, but also in violation of state law because they are operating as for-profit enterprises and they are not functioning as a primary caregiver," he said. (Reporting by Dan Whitcomb; Editing by Steve Gorman and Leslie Gevirtz)

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