16 letters were sent to pot shops or their landlords informing them that they are violating federal drug laws, even though medical marijuana is legal in California, according to the Associated Press.  The AP obtained copies of two letters sent by prosecutors.

The letters say if the dispensaries don't close, the shop or property owners will  face criminal charges and confiscation of their property.

Sacramento County is currently considering how best to regulate the shops, under the assumption that the federal government is not prosecuting dispensaries.

U.S. attorneys are scheduled to announce the crackdown at a  news conference on Friday.