The Obama administration is finally cracking down on the medical marijuana industry, in a big way.

In letters received by 16 licensed California dispensaries and their landlords this week, U.S. Attorneys threatened to swoop in and seize the properties if they don't close up shop within 45 days.

The Associated Press said that a coordinated crackdown on the medical marijuana industry would be announced at a press conference on Friday.

The move comes in the same week that the Internal Revenue Service took steps that may force Oakland's Harborside Health Center, the nation's largest medical marijuana dispensary, to shut down.

The same enforcement tactic that's being used against Harborside -- a very old law that prohibits groups that traffic in controlled substances from taking tax deductions -- could also be used against pot shops in all of the 16 states that have legalized the drug's use for medical purposes.

The U.S. medical marijuana market has grown into a $1.7 billion industry, and experts say it could potentially double that figure within just five years.

Photo: Flickr user digitalshay.