Federal prosecutors in California are cracking down on some of the state's medical marijuana dispensaries, signaling an escalation of the ongoing conflict between the U.S. government and the nation's burgeoning medical marijuana industry.

The four U.S. attorneys in California, the first state to pass a law legalizing marijuana use for patients with doctors' recommendations, have scheduled a joint news conference Friday where they plan to "outline actions targeting the sale, distribution and cultivation of marijuana."

Their offices refused to provide details in advance what moves the officials are taking or how many of the state's hundreds of storefront pot shops would be affected. But at least 16 pot shops or their landlords received letters this week warning face they would face criminal charges and confiscation of their property if the dispensaries do not shut down in 45 days.

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," according to the letters signed by U.S. Attorney Laura Duffy in San Diego. "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. For two years before that, 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.

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 appeared to be the most far-reaching effort so far to put that guidance into action.

Greg Anton, a lawyer who represents dispensary Marin Alliance for Medical Marijuana, said its 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 14-year-old 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.

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 W. 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. "By and large, they were empty threats, but they relied on them and the cost of postage to shut down as many facilities as they could without having to engage in criminal enforcement activity."

The San Diego medical marijuana outlets put on notice were the same dozen that city officials sued last month for operating illegally, after activists there threatened to force an election on a zoning plan adopted to regulate the city's fast-growing medical marijuana industry, City Attorney Jan Goldsmith said. A judge on Wednesday ordered nine of the targeted shops to close, while the other three shut down voluntarily, Goldsmith said.

Duffy, the U.S. attorney for far Southern California, planned to issue warning letters to property owners and all of the 180 or so dispensaries that have proliferated in San Diego in the absence of compromise regulations, according to Goldsmith.

"The real power is with the federal government," he said. "They have the asset forfeiture, and that means either the federal government will own a lot of property or these landlords will evict a lot of dispensaries."