SAN FRANCISCO — Federal prosecutors have launched a crackdown on pot dispensaries in California, warning the stores that they must shut down in 45 days or face criminal charges and confiscation of their property even if they are operating legally under the state’s 15-year-old medical marijuana law.

In an escalation of the ongoing conflict between the U.S. government and the nation’s burgeoning medical marijuana industry, at least 16 pot shops or their landlords received letters this week stating they are violating federal drug laws, even though medical marijuana is legal in California. The state’s four U.S. attorneys were scheduled Friday to announce a broader coordinated crackdown.

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.

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.

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.”

Laura 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 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.”