U.S. Attorney General Jeff Sessions warned Chicago and three other cities to get into compliance with federal laws or risk losing federal grants.
The section Sessions references, Section 1373, is a federal statute barring local and state governments from limiting communication regarding residents' immigration or citizenship status with federal officials.
Attorney General Jeff Sessions said in a written statement that sanctuary cities "adopt the view that the protection of criminal aliens is more important than the protection of law-abiding citizens and of the rule of law".
The Department also sent letters to officials in New Orleans, New York City and Philadelphia.
Sessions subsequently issued a memo that limited the categories of federal funding that would be in jeopardy to DOJ or Department of Homeland Security grants.
The U.S. Department of Justice has put five U.S. jurisdictions on notice: end sanctuary policies by October 27.
"Chicago will not be blackmailed into changing our values, and we are and will remain a welcoming city", Mayor Rahm Emanuel said at a press conference announcing that suit.
Additionally, the Justice Department said subsequent investigations found Miami-Dade County, Fla., and Clark County, Nev., where Las Vegas is located, were already in accordance with federal immigration law, though previously they had been listed otherwise.
Some cities say they will only honor detainers accompanied by criminal warrants, and that compliance with the requests is voluntary and not required under the statute.
Sessions' Justice Department made a point of noting the feds found no evidence of non-compliance with Wisconsin's Milwaukee County, along with CT.
In April, a presidential executive order that would have cut off funds to sanctuary cities was stymied by a judge's order.
Several U.S. cities and counties passed local laws or enacted procedures that prohibit law enforcement from notifying Homeland Security when an undocumented immigrant is identified or arrested.
"I commend the Milwaukee County Sheriff's Office and the State of CT on their commitment to complying with Section 1373", Sessions said, "I urge all jurisdictions found to be out of compliance in this preliminary review to reconsider their policies that undermine the safety of their residents".
The Kenney administration has until October 27 to submit a response to the DOJ assessment before a final determination is made, Hanson wrote.