Federal officials are warning some California medical marijuana dispensaries they must shut down within 45 days or face criminal prosecution and having their property confiscated, according to the Associated Press.

Marijuana remains illegal under federal law, despite state laws in California, Maine and several other states allowing sale and use for medicinal puproses. Maine has issued permits for eight dispensaries statewide based in part on California’s much larger network of medical marijuana dispensaries.

California’s four U.S. attorneys sent letters Wednesday and today notifying at least 16 pot shops or their landlords that they are violating federal drug laws, even though medical marijuana is legal in California. The attorneys are to announce their coordinated crackdown at a Friday news conference. The letters say that federal law “takes precedence over state law and applies regardless of the particular uses for which a dispensary is selling pot,” according to the AP.

The latest conflict between federal and state marijuana laws appears to be about the proximity of dispensaries to schools and other public facilities. California requires dispensaries to be at least 600 feet from schools. Maine has a 500-foot limit.

A federal anti-drug law, however, imposes additional penalties for sales of illegal drugs within 1,000 feet of schools, colleges, playgrounds, teen centers and other things.

So far, the initiative appears to be limited to California.

It’s not clear if any of Maine’s eight dispensaries are, or will be, located within 1,000 feet of such facilities. And Maine’s U.S. Attorney’s Office does not appear to be taking any measurements.

“U.S. Attorney Delahanty indicated that no such issues have been brought to the attention of the U.S. Attorney’s Office in this district,” said Assistant U.S. Attorney Donald Clark.