The Justice Department today announced that it is targeting dozens of medical-marijuana operations that it claims are fronts for illegal drug dealing and are not legal under the state's 15-year-old Compassionate Use Act.

Update at 5:20 p.m. ET: Federal prosecutors said at a news conference that the crackdown is directed at the "commercial marijuana industry," particularly operations near schools, parks and athletic fields.

"We want to put to rest the notion that large marijuana businesses can shelter themselves under state law," said U.S. Attorney Melinda Haag, the top federal prosecutor in San Francisco. She joined U.S attorneys from Sacramento, San Diego and Los Angeles in Sacramento.

Haag said the 1996 voter-approved law "has been hijacked by profiteers ... using the cover to make enormous amounts of money."

In a separate statement, the U.S. attorney for the central district of California, André Birotte Jr., said: "It is important to note that for-profit, commercial marijuana operations are illegal not only under federal law, but also under California law. While California law permits collective cultivation of marijuana in limited circumstances, it does not allow commercial distribution through the store-front model we see across California."

Criminal complaints targeted operators or landlords across California and against a Florida man who moved to the state to allegedly grow and distribute marijuana.

The Justice Department also issued warning letters to 38 medical-marijuana dispensaries, the Los Angeles Times says.

In Northern California, the Justice Department has ordered landlords to evict at least four Bay Area outlets within 45 days or face prosecution, The Bay Citizen reports.

The Sacramento Bee has more details from today's news conference.

Update at 7:36 a.m. ET: 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 today to "outline actions targeting the sale, distribution and cultivation of marijuana."

Their offices refused to provide details in advance of what moves the officials are taking or how many of the state's hundreds of storefront pot shops would be affected, the Associated Press reports.

Initial stories indicated that all medical-marijuanja dispensaries in California had been ordered to shut down, but the actual target remains unclear.

At least 16 pot shops or their landlords received letters this week warning they would face criminal charges and confiscation of their property if the dispensaries do not shut down in 45 days, the AP reports.

The news agency 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."

The Los Angeles Times says that one letter to the Marin Alliance notes that the dispensary is within a prohibited distance of a park, raising the possibility that enforcement will zero in on stores near schools and playgrounds.

The newspaper, quoting an unidentified source, says the U.S. prosecutor in Los Angeles would not initially focus on dispensaries in that city. The Times also quotes Dale Gieringer, the director of California NORML, which backs legalizing marijuana, as saying the crackdown apparently will be tailored to fit the regional differences in the state.

"They want to do a clean sweep in San Diego, whereas in Northern California they can't possibly do a clean sweep," he tells the newspaper. "There's no political support for it. It would be devastating."

Original post: The Obama administration has ordered all medical-marijuana dispensaries in California to shut down within 45 days or they will be prosecuted and have their property confiscated for violating federal drugs laws, the Associated Press reports.

Even licensed outlets operating under the 1996 voter-approved medical-marijuana law are subject to the federal crackdown. California's four U.S. attorneys plan an announcement Friday.

AP says the federal prosecutors have notified at least 16 pot shops or their landlords.