U.S. Attorney John Walsh sent a letter Friday to a lawyer representing medical marijuana dispensaries, saying the safe harbor doesn't exist because marijuana remains illegal under federal law.

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 A letter sent to one of the dispensaries was later withdrawn because the school building near it isn't used for educating children.

A lawyer representing other dispensaries then asked Walsh where the line is drawn on federal enforcement in Colorado shops. The lawyer had concluded that as long as shops were not near schools, they were safe from prosecution.