Federal prosecutors have launched a crackdown on California medical marijuana dispensaries, but it's unclear whether any pot clubs in Sonoma County are targeted.

The enforcement push appears to be aimed at large-scale operators, said William Panzer, an Oakland attorney who represents Marvin's Gardens Herbal Cooperative, a Guerneville dispensary.

U.S. attorneys sent letters this week to at least 16 pot clubs or landlords warning them to shut down in 45 days or face criminal charges and confiscation of their property, even if they are operating under the state's 15-year-old medical marijuana law.

Marvin's Gardens wasn't one of them, Panzer said.

In an escalation of the ongoing conflict between the U.S. government and the nation's burgeoning medical marijuana industry, the prosecutors said they are violating federal drug laws, even though medical marijuana is legal in California.

The state's four U.S. attorneys are scheduled to announce a broader coordinated crackdown at a Friday news conference.

Their offices refused to confirm the closure orders. The Associated Press obtained copies of the letters that a prosecutor sent to at least 12 San Diego dispensaries. They state that federal law "takes precedence over state law and applies regardless of the particular uses for which a dispensary is selling and distributing marijuana.

"Under United States law, a dispensary's operations involving sales and distribution of marijuana are illegal and subject to criminal prosecution and civil enforcement actions," letters signed by U.S. Attorney Laura Duffy in San Diego read.

"Real and personal property involved in such operations are subject to seizure by and forfeiture to the United States ... regardless of the purported purpose of the dispensary."

The move comes a little more than two months after the Obama administration toughened its stand on medical marijuana following a two-year period during which federal officials had indicated they would not move aggressively against dispensaries in compliance with laws in the 16 states where pot is legal for people with doctors' recommendations.

A directory of medical marijuana dispensaries shows a dozen in Sonoma County, mostly in unincorporated territory. Santa Rosa and Sebastopol each have one marijuana club.

The federal government seems to be pressuring large-scale dispensaries, said Panzer, who co-authored Proposition 215, the 1996 ballot initiative that legalized medical marijuana in California.

Earlier this year, a federal memorandum suggested prosecutors would target distributors selling at least 200 kilograms of marijuana a year and dispensaries that are near schools and parks. They would<NO1><NO> also target pot clubs violating state or local laws or using profits to fund other criminal activities.

"It's very possible we are going to see a shift from large centralized dispensaries to smaller patient cooperatives," Panzer said.

Lisa Gygax, a Forestville attorney who represents Organic Cannabis Foundation on Todd Road, said the dispensary hasn't received one of the warning letters.

But the threat seems serious, she said. "They're trying to scare everybody into closing," Gygax said. "I believe this is the new federal blitzkrieg. They are really going to take some people down."

The crackdown is a disappointing turnabout for President Obama, she said. "It's the most outrageous betrayal of a voting bloc that Obama has done," Gygax said.

Dan Beck, a Santa Rosa attorney who represents three dispensaries, said his clients have not <NO1><NO>received letters<NO1><NO>. But he said the federal campaign "will have a chilling effect" on locally <NO1><NO>regulated pot clubs. "It's going to cause landlords and property owners not to rent or sell to them," he said.

Sonoma County has issued permits for a number of medical marijuana dispensaries, but it has no say in enforcement of federal drug laws, said County Counsel Bruce Goldstein. "It probably wouldn't be in our jurisdiction," he said.

The Department of Justice issued a policy memo to federal prosecutors in late June stating that marijuana dispensaries and licensed growers in states with medical marijuana laws could face prosecution for violating federal drug and money-laundering laws. The effort to shutter California dispensaries appears to be the most far-reaching effort so far to put that guidance into action.

"This really shouldn't come as a surprise to anyone. The administration is simply making good on multiple threats issued since President Obama took office," said Kevin Sabet, a former adviser to the president's drug czar who is a fellow at the University of Pennsylvania's Center for Substance Abuse Solutions.

"The challenge is to balance the scarcity of law enforcement resources and the sanctity of this country's medication approval process. It seems like the administration is simply making good on multiple statements made previously to appropriately strike that balance."

Greg Anton, a lawyer who represents a Marin Alliance for Medical Marijuana, said the 14-year-old dispensary's landlord received an "extremely threatening" letter Wednesday invoking a federal law that imposes additional penalties for selling drugs within 1,000 feet of schools, parks and playgrounds.

The landlord was ordered to evict the pot club or risk imprisonment, plus forfeiture of the property and all the rent he has collected while the dispensary has been in business, Anton said.

The Marin Alliance's founder "has been paying state and federal taxes for 14 years, and they have cashed all the checks," he said. "All I hear from Obama is whining about his budget, but he has money to do this which will actually reduce revenues."

Kris Hermes, a spokesman for the medical marijuana advocacy group Americans for Safe Access, said the warnings are part of what appears to be an attempt by the Obama administration to curb medical marijuana on multiple fronts and through multiple agencies. A series of dispensary raids in Montana, for example, involved agents from not only the FBI and U.S. Drug Enforcement Agency, but the Internal Revenue Service and Environmental Protection Agency.

Going after property owners is not a new tactic, though, Hermes said. Five years ago, the Department of Justice under President George Bush made similar threats to about 300 Los Angeles-area landlords who were renting space to medical marijuana outlets, some of whom were eventually evicted or closed their doors voluntarily, he said.

"It did have an impact. However, the federal government never acted on its threats, never prosecuted anybody, never even went to court to begin prosecutions," Hermes said.

Press Democrat Staff Writer Steve Hart and The Associated Press contributed to this story.