No federal appeals court has ever described an ICE detainer as anything but a request. The Third Circuit said in 2014 (Galarza v. Szalczyk), after summarizing the position of the various circuits:

Reading § 287.7 [the Detainer provision]to mean that a federal detainer filed with a state or local law enforcement agency was a command to detain an individual on behalf of the federal government would violate the anti-commandeering doctrine of the Tenth Amendment.

The ACLU, which represented Galarza has more here.

Under 8 C.F.R. § 287.7, a detainer is a tool used by ICE to hold individuals who are taken into state custody. It is a written request that the state law enforcement officers detain an individual for no more than 48 hours after his or her release on state charges so that ICE can investigate that person's immigration status. In 2016, the federal court for the Northern District of Illinois held that nearly all ICE detainers issued by the Chicago Field Office were invalid.

If ICE wants custody of someone in a local jail, it should get an arrest warrant.

Here's a map of the local jurisdictions refusing to grant ICE detainer requests in the wake of the multiple federal court rulings. Here is a Feburary, 2017 legal update on court rulings holding ICE detainer requests and merely requests (and in some cases holding local jurisdictions financially accountable for violating constitutional rights when then comply.)

Just last month, a Rhode Island federal court refused to grant the immunity to an ICE agent and to the ICE Field Office Director for Boston in a case where a U.S. citizen was improperly detained purusant to an invalid ICE detainer. (Morales v. Chadbourne.)

Ada Morales was born in Guatemala, and became a naturalized United States citizen on September 11, 1995 under her maiden name, Ada Amavilia Cabrera. She has a social security number and a United States passport. Despite this, Ms. Morales was held at the state prison on an Immigration and Customs Enforcement ("ICE") detainer that was issued solely based on her Hispanic last name and her Guatemalan birthplace. This twenty-four hour illegal detention revealed dysfunction of a constitutional proportion at both the state and federal levels and a unilateral refusal to take responsibility for the fact that a United States citizen lost her liberty due to a baseless immigration detainer through no fault of her own. There is plenty of blame to go around amongst the Defendants in this case.

Some of the detainer requests in today's report were issued in 2014 - 2016. I doubt many of the subjects were still in custody this month when the detainer requests were denied.

Here's a fact sheet on so called "sanctuary cities." Nearly 300 law professors have joined a letter saying Trump's executive order is unconstitutional.

The only shame here, as usual, is on Donald Trump, for issuing the order.