The highest court in Massachusetts ruled Monday that local law enforcement cannot keep people in custody solely at the request of federal Immigration and Customs Enforcement. The practice, often known as an "ICE detainer," enabled federal authorities to take a longer look at the immigration status of people whom they suspect might be in the country illegally, even if they were otherwise free to leave.

"This could mean the individual's charges have been dismissed, they've posted bail or their jail sentence has been completed," Shannon Dooling of member station WBUR explains. "The detainer — which is not the same as an arrest warrant, which requires proof of probable cause and a judge's signature — gives ICE up to two days to look into a person's immigration status and potentially pursue deportation."

But the state's laws provide "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," the Supreme Judicial Court said in its unanimous decision.

And because deportation is a civil process, "not a criminal prosecution," the court appeared skeptical that state police — not just court officers — could accede to an ICE detainer either: "Conspicuously absent from our common law is any authority (in the absence of a statute) for police officers to arrest generally for civil matters, let alone authority to arrest specifically for Federal civil immigration matters."

Monday's verdict came in Commonwealth v. Lunn, which sought to determine the fate of Sreynuon Lunn, an immigrant who had been held in a courthouse holding cell after a charge of unarmed robbery against him had been dismissed. "Born to Cambodian parents in a Thai refugee camp, Lunn isn't recognized as a citizen by either country and both refuse to take him," WBUR reported last month, noting he was first ordered deported from the U.S. nearly a decade ago.

Still, when Lunn was seven months old "he was legally allowed into the country as a refugee and given lawful permanent resident status," The Associated Press notes. "He has two U.S.-born children."

Civil liberties activists hailed the court's decision.

"This court decision sets an important precedent that we are a country that upholds the constitution and the rule of law," Carol Rose, executive director of the ACLU of Massachusetts, said in a statement. "This victory is the first of its kind in the nation."

"This is hopefully something that we will look back on and see that it sent a message out to all the other states that this is not the job of the states, to be enforcing federal immigration laws," Susan Church, former head of the New England chapter of the American Immigration Lawyers Association, told WBUR.

Federal authorities did not immediately issue a public response to the decision.

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