MILWAUKEE (CBS 58) -- Federal officials are "seeking assurances" Milwaukee will not enforce a new ordinance banning ICE agents from wearing masks. 

CBS 58 obtained a copy of the letter Friday, which was sent to Mayor Cavalier Johnson and the city attorney's office. 

The Department of Justice points to the Supremacy Clause in the U.S. Constitution, arguing that keeps federal officers from having to follow any local laws that clash with federal policy. 

Last week, local leaders said they were exploring legal action after a surge in recent ICE activity in Wisconsin. That included immigration officers using county parks and a Milwaukee police station as staging areas. 

A spokesperson for Mayor Johnson says they have yet to receive a copy of the letter. 

A response from the mayor's office Friday said:

"The Mayor will leave the specific legal issues to the City Attorney. He does, however, maintain some actions and behavior of Federal immigration agents have been inappropriate and dangerous. Federal agents are undermining the trust our resident have with their aggressive enforcement efforts."