Federal prosecutors in California Friday announced a crackdown on what they called an explosion of illegal commercial marijuana dispensaries.

All four of California's U.S. attorneys said that since a 2009 U.S. Justice Department memo gave prosecutors leeway to avoid going after medical marijuana users who comply with state laws, illegal operations that allegedly game the system have proliferated.

The prosecutors held a press conference Friday after their offices sent letters warning landlords about renting space to retail marijuana outfits or risk seizure.

We are not focused on backyard growth of marijuana for sick people to use, said Benjamin B. Wagner, the U.S. attorney for California's Eastern District in Sacramento. Illegal operations often use the trappings of state law for cover, but in fact are abusing state laws.

Golden State: First To Legalize Medical Marijuana

California, in 1996, became the first state to legalize medicinal marijuana. Patients with a valid doctor's recommendation or their caregiver can grow and possess marijuana.

Again, we're not after people who are sick or their primary caregivers, Wagner said. That's not our focus. We have limited resources.

The Obama administration in 2009 sent a memo to U.S. attorneys that said their core priority is to prosecute significant drug traffickers, providing leeway to avoid going after individuals who comply with state medical marijuana laws.

The federal prosecutors said that this new policy shift have allowed people to open up lucrative cross-state marijuana selling and growing operations under the guise of state-sanctioned medicinal marijuana outfits.

The U.S. attorney stressed that his office will enforce federal laws against large marijuana growing and selling operations, as well as dispensaries that are near schools, parks and other areas with children.

[Federal law] does not allow this brick-and-mortar Costco model of dispensaries, said Andre Birotte Jr., U.S. attorney for the Central District based in Los Angeles.

Medical marijuana advocates criticized the enforcement actions on dispensaries, arguing that the Obama administration is on a full-scale assault on medical marijuana patients'... ability to access medicine, according to a statement from the Drug Policy Alliance.

Instead of encouraging state and local authorities to regulate medical marijuana distribution in the interests of public safety and health, said Ethan Nadelmann, executive director of the Drug Policy Alliance, his administration seems determined to re-criminalize as much as possible.