U.S. Attorney General Eric Holder has told Kansas Gov. Sam Brownback that a new state law attempting to block federal regulation of some guns is unconstitutional.

The U.S. attorney’s office for Kansas on Thursday released a copy of a letter to Brownback from Holder last week, sent the day after the new state law took effect.

The law says the federal government has no authority to regulate guns manufactured, sold and kept only in Kansas and makes it a felony for a federal agent to enforce any regulations of those guns.

Holder said the U.S. Constitution prohibits the state from pre-empting federal laws. He said the federal government is willing to go to court.

Aides to Brownback and Kansas Attorney General Derek Schmidt did not immediately return messages seeking comment.