The department said New Orleans, New York City, Philadelphia, and Cook County in IL along with its largest city Chicago, "have preliminarily been found" to have policies that violate the law.
Offending cities and states stand to lose money from the Edward Byrne Memorial Justice Assistance Grant (or JAG grant), which provided Philadelphia with $1.6 million in 2016 to spend on police overtime, training, equipment, courtroom technology and other aspects of the criminal justice system. The law cited by the feds in the 2016 report, the Connecticut Trust Act, stipulates the conditions when law enforcement should detain someone for violating federal immigration law but does not prohibit the sharing of any information with Immigration and Customs Enforcement.
A letter from the Justice Department released Thursday said officials believe NY is violating a law requiring cooperation on immigration enforcement - one of four cities put on notice they were out of compliance.
The U.S. Department of Justice has put five U.S. jurisdictions on notice: end sanctuary policies by October 27.
The determinations came after the Justice Department earlier in the year had asked several local jurisdictions to detail their compliance with the law in order to make a determination about their eligibility for certain federal grants.
Sessions and President Donald Trump have been vocal opponents of so-called "sanctuary cities", which enact policies to protect undocumented immigrants.
Although the sanctuary city debate has boiled over in the age of Trump, it is a long-standing fight. It determined Milwaukee County, Wisconsin, and CT to be in compliance. "The NOPD will continue to focus on the arrest and conviction of violent criminals, regardless of their immigration status".
The police memo, dated May 17, 2001, is created to encourage immigrants to utilize city services without fear of reprisal.
New Orleans officials are holding a news conference Thursday to discuss the city's compliance with federal immigration law. U.S.C. 1373 - states that a federal, state or local government entity can not restrict any government entity from "sending to, or receiving from, the Immigration and Naturalization Service information regarding the citizenship or immigration status, lawful or unlawful, of any individual".
The DOJ claims that an NOPD policy "may" violate a federal statute that deals with how local governments provide information to federal immigration officials "depending on how your jurisdiction interprets and applies them", Alan Hanson, acting assistant attorney general wrote.
The city has said its policies are legal, and vowed to sue if money is actually taken away. But his office did not immediately comment on the letter.
Mayor de Blasio has defended the sanctuary city policy saying he won't turn the NYPD into a deportation force that breaks apart families.