Letters sent to the dispensaries by the U.S. attorney's office gave them 45 days to close or face criminal prosecution and asset forfeiture, The Denver Post reported.
Although the state's medical marijuana dispensaries operate in violation of federal law, federal authorities in Denver have never before prosecuted a medical-marijuana business owner operating in clear compliance with state's more lenient laws, the newspaper said.
The federal action is the latest in a tug-of-war between federal and state authorities over the issue of medical marijuana.
"Our interest is to have these stores moved away from schools," Colorado U.S. attorney's spokesman Jeff Dorschner said. "Our first step is protecting children who go to school near these marijuana stores."
Nicholas King, president of the Association of Cannabis Trades for Colorado, called the crackdown a violation of states' rights.
Colorado law specifies dispensaries must be at least 1,000 feet from schools
"We do have a highly regulated system that is being put in place," King said. "And to come in and use this as a justification is not appropriate. This is really a state issue, a zoning issue. It's not really a federal issue as far as we're concerned."