The 23 marijuana dispensaries located near schools in Colorado that received warning letters last week telling them they must shut down should take the warning seriously, according to Colorado U.S. Attorney John Walsh.

He told the Associated Press the letters informed the dispensaries and their landlords they have until February 27 to shut down, move or face federal penalties. Owners of the dispensaries and the property on which they are located could lose their assets and property. He added that criminal prosecution is also possible.

He disputed the claim that the federal government has shown its tacit support of Colorado’s marijuana industry by not increasing enforcement. “We haven’t been sitting by. We’ve been taking marijuana enforcement action,” Walsh told the AP.

Last year, U.S. attorneys sent warning letters to several states about medical marijuana laws. The letters indicated that people involved in the growing, dispensing and regulating of medical marijuana have the potential to be prosecuted—even if they are following state laws. The letters were sent to officials in Colorado, Washington, California, Montana and Rhode Island.

Walsh says some surveys suggest medical marijuana has contributed to an increase in drug use among teenagers.