The Massachusetts Supreme Judicial Court ruled Monday that local law enforcement does not have the authority to honor Immigration and Customs Enforcement (ICE) detainer requests, a key tool used to detain and deport undocumented people.

Lunn v. Commonwealth was argued at Massachusetts’ highest court on April 4 and addressed “whether local and state officials who hold undocumented immigrants on ‘ICE detainers’ are in violation of the Massachusetts state constitution and the United States constitution,” according to the American Civil Liberties Union (ACLU).

The court concluded that state law “provides no authority for Massachusetts court officers to arrest and hold an individual solely on the basis of a Federal civil immigration detainer, beyond the time that the individual would otherwise be entitled to be released from State custody.”

Monday’s decision dealt a serious blow to the Trump administration’s fight against so-called sanctuary cities, jurisdictions that refuse to always honor ICE detainer requests. The administration has attempted to pull federal funding from these jurisdictions, while spinning the false narrative that these cities are releasing “dangerous criminals” into the public.

President Donald Trump has focused heavily on ICE detainer requests, asserting that law enforcement agencies should be forced to honor them. What Trump routinely fails to mention is that all local law enforcement agencies—including those in so-called sanctuary cities—cooperate with ICE by forwarding the biometrics of undocumented immigrants to the agency.

As Rewire reported, so-called sanctuary cities offer few concrete protections to undocumented immigrants, operating largely as statements of solidarity. Jurisdictions have been instructed by courts not to honor ICE detainer requests in order to avoid due process violations and costly lawsuits such as Lunn’s.

At the center of the Massachusetts case was Sreynuon Lunn, born in a Thai refugee camp to Cambodian parents fleeing the Khmer Rouge. Lunn has had multiple interactions with the criminal justice system since 2008, when he was first ordered deported to Cambodia. Neither Cambodia nor Thailand, however, recognize him as a citizen.

Lunn has been in ICE custody multiple times, including in February when he was facing larceny charges. The case was dismissed, and Lunn was ordered released. But Massachusetts authorities held him for two days, as requested by an ICE detainer, giving the federal immigration agency time to take Lunn into custody.

Once in ICE custody, Lunn was unlawfully detained for three months and denied due process, according to his attorneys. In honoring ICE’s detainer request, the attorneys argue, Massachusetts violated a U.S. Supreme Court case that ruled a person cannot be detained indefinitely if their country of origin refuses admission.

Lunn’s case is a prime example of something called “crimmigration,” a term coined in 2006 by legal scholar Juliet Stumpf to describe the intersection of criminal law and immigration law. The overlapping of these two systems allows the federal government to detain people suspected of committing crimes and reduce the number of unauthorized immigrants in the country.

ICE detainer requests are a key tool of crimmigation. Made in writing, the request typically asks “that a local jail or other law enforcement agency detain an individual for an additional 48 hours (excluding weekends and holidays) after his or her release date in order to provide ICE agents extra time to decide whether to take the individual into federal custody for removal purposes,” the ACLU explains.

C.M. Cronen, the Boston ICE field office director, said in a statement that Monday’s ruling “weakens local law enforcement agencies’ ability to protect their communities.” The agency is reviewing the decision to determine next steps.

Detainer requests are not mandatory or legally required, and local and state law enforcement agencies have no obligation to honor them. A detainer does not indicate probable cause and it does not operate as an arrest warrant.