Frustrated by federal inaction in enforcing immigration laws and beset by public safety threats from unchecked immigration across its border with Mexico, the State of Arizona enacted its own set of laws to try to cope with some of the problems. One provision, section 2(b) of the Act known as S.B. 1070, requires law enforcement officers in Arizona to make "a reasonable attempt to determine the immigration status” of any person they stop, detain, or arrest on some other legitimate basis, if “reasonable suspicion exists that the person is an alien and is unlawfully present in the United States."
The United States sued to prevent the state from enforcing this provision (among others), arguing that the entire field of immigration enforcement was preempted from state control by federal law. The United States also argued that immigration checks could unlawfully prolong otherwise lawful detentions. On June 25, 2012, the U.S. Supreme Court rejected both of these contentions.
As to federal preemption, the court pointed out that various provisions of federal law specifically authorize mutual communication between local and federal officers on immigration matters. Quoting a lower court opinion, the court said that "Arizona officers have authority to enforce the criminal provisions of federal immigration law. If § 2(b) only requires state officers to conduct a status check during the course of an authorized, lawful detention or after a detainee has been released, the provision likely would survive preemption."
On the issue of whether state officers could routinely conduct immigration checks during lawful detentions, the court cited to its earlier decision in Muehler v. Mena, which had already decided this issue, "finding no violation where questioning about immigration status did not prolong a stop." In the absence of evidence that Arizona officers would be applying § 2(b) in such a way as to cause detentions to be unlawfully prolonged, the Supreme Court saw no reason to invalidate this particular provision. (Other unrelated provisions were found to be preempted by federal law.)
Recapitulation