Federal prosecutors in Colorado made their strongest move to date against the state’s medical-marijuana industry Thursday, sending letters to 23 dispensaries ordering them to close.

Each dispensary is located within 1,000 feet of a school, and the letters represent a sharp tug to rein in medical-marijuana businesses, all of which operate in violation of federal law. The U.S. attorney’s office told the dispensaries’ owners in the letters that if they do not close within 45 days, they face criminal prosecution and asset forfeiture.

Federal authorities in Denver have never before prosecuted a medical- marijuana dispensary owner operating in clear compliance with state law.

“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.”

Letters also went out to property owners who lease space to the dispensaries, warning the owners that their buildings could be seized.

University of Denver law professor Sam Kamin said the limited crackdown fits in with a series of federal actions nationwide that seek to restrict — but not immediately eliminate — medical-marijuana businesses.

“It is a tightening of the screws,” Kamin said.

Under that new pressure, medical-marijuana advocates Thursday had mixed reactions.

Nicholas King, president of the Association of Cannabis Trades for Colorado, said the crackdown is a violation of states’ rights. King also owns the Alpine Herbal Wellness dispensary in Cherry Creek, which is a couple blocks from Bromwell Elementary School.

King said he expects to receive one of the letters and said he doesn’t know yet whether he will move. He said the dispensary is far enough from the school that children do not walk by regularly, and he said his store is professional and blends in with the neighborhood.

“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.”

The Medical Marijuana Industry Group, perhaps the most influential dispensary- lobbying organization in the state, didn’t directly address the letters, instead saying that it is working on an education campaign to make sure medical marijuana isn’t being given to kids.

“We fully support keeping regulated substances out of the hands of unauthorized users and schools,” the group said in a statement.

Colorado law specifies that dispensaries must be at least 1,000 feet from schools but also allows local governments to shrink that distance or grandfather in existing dispensaries. The 1,000-foot buffer is also significant because it triggers enhanced penalties under federal law.

Colorado has about 700 dispensaries, according to the most recent count of businesses that had an active license application pending with state regulators. An analysis by the I-News Network, an independent investigative reporting outlet, found as many as 36 dispensaries statewide that could be considered within 1,000 feet of a school.

More than a dozen of those are in Denver, and Denver Public Schools spokeswoman Kristy Armstrong said the district agrees with the crackdown.

“We are pleased to see the U.S. attorney’s office is enforcing the buffer zone,” she said.

Congressman Jared Polis, who has defended Colorado medical-marijuana laws in Washington, also expressed support for abiding by the 1,000-foot buffer in state law.

“The Justice Department has repeatedly made clear that dispensaries that are in compliance with state law are not an enforcement priority,” Polis, a Democrat from Boulder, said in a statement.

The U.S. Justice Department has previously said it would not target people operating in compliance with state medical-marijuana law. It then clarified that policy over the summer to state the exemption doesn’t apply to dispensaries and their owners.

John Ingold: 303-954-1068 or jingold@denverpost.com