DENVER—The U.S. attorney in Colorado says there is no such thing as "safe harbor" for pot shops.

U.S. Attorney John Walsh sent a letter Friday to a lawyer representing medical marijuana dispensaries, saying safe harbor doesn't exist for such shops because marijuana remains illegal under federal law. It was the latest in an exchange of letters between Walsh and lawyer Robert Corry regarding the location of dispensaries and how far they need to be from schools.

Walsh said in the new letter that it is at his office's discretion to take enforcement action against any and all medicinal marijuana dispensaries.

"That's an awesome amount of power that the law does not intend to hand to a single federal prosecutor," Corry said Friday.

Corry had sent a letter to Walsh in an attempt to pin down where the line is drawn on federal enforcement in Colorado shops.

On Jan. 12, Walsh told 23 Colorado dispensaries within 1,000 feet of a school to close by Feb. 27 or face penalties. A letter sent to one of the dispensaries was later withdrawn because the school building near it isn't used for educating children. All the shops met deadline and closed without incident.

Corry concluded that as long as shops were not near schools, they were safe from prosecution.

Walsh responded Friday that advising clients that there is a safe harbor is "incorrect and untruthful, and would mislead them, factually and legally."

Jeff Dorschner, a spokesman for the U.S. attorney's office, said there are other factors that can close down shops like unlawful use or possession of a firearm and having amounts of marijuana not in compliant with state and local law.

Possible civil or criminal remedies include seizing the building where a shop operates and jail time for shop keepers.

Dorschner said it's not possible to answer whether a shop in compliance with state rules and regulations and not located near a school would still face any trouble.

Corry said the U.S. attorney's letter regarding no safe harbors allows an air of uncertainty among medicinal marijuana workers. The uncertainty brings an "entire economic sector of Colorado on its knees," Corry said.

Corry said he plans to draft a response to Walsh's latest letter clarifying under what federal laws the shops can operate.