The Justice Department took its first steps on Friday to strip Chicago and Cook County of some crime fighting grants as part of a drive to deny federal money to so-called "sanctuary cities" shielding illegal immigrants.
The letters warn officials they must provide proof from an attorney that they are following the law or risk losing thousands of dollars in federal grant money that police agencies use to fund anything from body cameras to bulletproof vests.
Attorney General Jeff Sessions has increasingly warned that the administration will punish communities that refuse to cooperate with efforts to find and deport immigrants in the country illegally.
That law states that the officials must attest that a "Federal, State or local government entity or official may not prohibit, or in any way restrict, any government entity or official from sending to, or receiving from, the Immigration and Naturalization Service information regarding the citizenship or immigration status, lawful or unlawful, of any individual".
In a statement issued on Friday, the Justice Department said the recipients of its letters were "crumbling under the weight of illegal immigration and violent crime".
The municipalities that received the letters were identified in a Justice Department inspector general's report previous year as potentially out of compliance with the requirements.
FILE - In this January 25, 2017 file photo, a woman holds a sign at a rally outside of City Hall in San Francisco.
Alan Hanson, the acting assistant attorney general for the Justice Department's grant-making arm, warned the cities that they're required to submit proof they comply with federal immigration law.
It also pointed to a Milwaukee County rule that immigration detention requests be honored only if the person has been convicted of one felony or two misdemeanors, has been charged with domestic violence or drunken driving, is a gang member or is on a terrorist watch list, among other constraints.
Earlier this week, Mr Sessions accused sanctuary cities of undermining law enforcement efforts to fight transnational street gangs.