DENVER (CBS4) – Federal authorities are planning to crack down on the medical marijuana business in Colorado on a large scale for the first time.

Warning letters will be going out to dispensaries and grow facilities near schools, CBS4 investigator Rick Sallinger has learned. So far it’s not clear how soon that will happen.

Dispensaries that receive the letters will be given 45 days to shut down or move operations. If they don’t comply, they will be shut down by the U.S. attorney in Colorado.

The dispensaries who are set to be targeted are the ones that are located within 1,000 feet of schools. That measurement is being used because that distance already appears in federal law as a factor in drug crime sentencing.

The move comes after the Justice Department sent out a memo clarifying that marijuana has been and remains illegal under federal law despite what has taken place with state regulations. Colorado is one of 16 states where medical marijuana laws have been approved.

RELATED STORY: Justice Department Memo Writer Is Mum On Pot Crackdown

Many of the state’s dispensaries that are closer than 1,000 feet to a school have already been approved to be there under local laws. They usually have been grandfathered in.

That’s the case with the dispensary Rocky Mountain High at 3rd and University in Cherry Creek North. It’s located within of Bromwell Elementary School, but was grandfathered in under Denver municipal laws.

Bromwell Principal Jody Cohn told Sallinger she thinks a crackdown on the dispensary is “vital for the safety and the security of our students and our community.”

CBS4 found several other examples of dispensaries that are clearly within 1,000 feet of a school in Denver. One called Med-Stop that was also approved by the city is diagonally across 8th Avenue from Del Pueblo Elementary School. A measurement by Sallinger showed is was only 161 feet away. Two dispensaries were also spotted right down the block from Morey Middle School.

Robert Corry, an attorney who represents dispensaries, said medical marijuana operations are now strictly regulated under Colorado state laws.

“The federal apparatus here has better things to do,” said Corry. “My reaction would be the federal government is essentially declaring war on the voters of our state (who) passed a Constitutional amendment.”

U.S. attorneys in California recently announced in a separate medical marijuana crackdown that they would be targeting landlords who rent retail space to dispensaries, as well as dispensary owners themselves.

While Med-Stop on 8th Avenue in Denver is across the street from Del Pueblo School, that facility is now used only for administrative purposes. The Denver School for International Studies is located nearby on 6th Avenue. It is not clear if Med-Stop would qualify for an exemption from any federal enforcement action.