Gov. Jay Inslee signed the measure Tuesday implementing the new rules, which rank among the strongest statewide mandates in the nation. California and Oregon have similar laws.
The move comes as President Donald Trump's administration has cracked down on immigration by increasing migrant detentions and attempting to cancel the Deferred Action for Childhood Arrivals program.
Under the new Washington state law, local law enforcement agencies are broadly prohibited from asking about immigration status or place of birth unless directly connected to a criminal investigation, and both local jails and state prisons are prohibited from complying with voluntary "immigration holds" requested by federal authorities, or from notifying federal authorities when an immigrant is about to be released from their custody.
"Our state agencies are not immigration enforcement agencies," said Inslee, a Democrat who is also running for president. "We will not be complicit in the Trump administration's depraved efforts to break up hard-working immigrant and refugee families."
Critics say sanctuary laws hamper the ability of police to do their job.
Republican state Sen. Phil Fortunato, who voted against the Washington law when it was before the Legislature, said he thought it would hamper the ability of police to take action against subjects they suspect to be illegal immigrants.
"This not only puts law enforcement at risk, it puts private citizens at risk," Fortunato said.
Jorge Baron, head of the Seattle-based Northwest Immigrants' Rights Project, said the rules would increase public safety by making migrants and their families less afraid to report crimes or come forward as witnesses.
Immigrants, shouldn't have to worry that police might demand "papers" at any moment, said Democratic state Sen. Lisa Wellman, the daughter of Jewish immigrants and the bill's sponsor.
"We have 30% of Microsoft here on visas," Wellman said earlier this year when the bill was being debated.
Under the law, police officers in Washington won't be able to inquire about immigration status except in limited circumstances, and the state attorney general will draw up rules for courthouses, hospitals and other state government facilities aimed at limiting their use as places where federal immigration agents look for people in the country illegally.
Local and state authorities are also broadly prohibited from sharing immigration information about people in custody with immigration authorities, except under a valid court order or where required by law.
A 2017 executive order from Inslee imposed similar requirements but only on state agencies, a move advocates said fell short of full sanctuary state status. Tuesday's measure expands the rules to include all local law enforcement.
That's significant because most police interactions tend to occur at the city and county level, rather than with state troopers, and because local police have long been targets for cooperation requests from federal immigration authorities, generating the majority of deportations, said Lena Graber, an attorney with the Immigrant Legal Resource Center, a national non-profit that tracks sanctuary policies.
The law, Graber said, gives Washington, "the strongest and most comprehensive state law on sanctuary in the country."
While the exact definition of "sanctuary state" is flexible, Graber said, five other states have seen state laws or executive orders or rules used to create statewide protections against coordination between local and federal law enforcement on non-criminal immigration investigations: Illinois, Connecticut, Rhode Island, New Jersey, and Vermont.
Oregon was the first state to adopt a statewide sanctuary policy, in 1987, a straightforward restriction on police spending any resources - including paid staff time - going after people who are in the country illegally but haven't committed other crimes.
California passed more robust protections in 2017, including yearly reporting requirements and a rule that authorities must get written consent from people they arrest before allowing them to be interviewed by immigration officers.
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