Along with the demand for proof, DoJ threatened to use its subpoena power to pry loose relevant documents.

The effort by the government to ratchet up pressure on sanctuary cities to comply with the law was intensified this past week when the Department of Justice sent a letter to several jurisdictions demanding that they prove they are cooperating with federal immigration authorities.

Fox News:

The department is further seeking a legal opinion on whether policies in Oakland's police manual violate the federal statute requiring information-sharing with federal immigration authorities. "When cities and states enact policies that thwart the federal government's ability to enforce federal immigration law, they choose to place the protection of criminal aliens over the safety of their communities," Attorney General Jeff Sessions said in a statement. "The Justice Department will not tolerate this intentional effort to undermine public safety and the rule of law, and I continue to remind all jurisdictions to reconsider policies that put their residents in harm's way." Seattle Mayor Jenny Durkan denounced the department's subpoena threat. "Our city complies with federal immigration law and asks that the Department of Justice and ICE [Immigration and Customs Enforcement] do the same," Durkan said in a statement Friday. "The federal government does not get to run our cities or convert our local law enforcement officials into immigration cops. I implore this administration to focus on real public safety threats, like the opioid crisis, instead of unnecessarily threatening our residents and mayors across the country."

Note the exaggeration and hyperbole: "The federal government does not get to run our cities or convert our local law enforcement officials into immigration cops." That is total, complete nonsense and amounts to political posturing, not a legitimate response to a reasonable request from Washington.

“Unlike some in the Trump administration, Seattle respects the rule of law,” the mayor, a former U.S. attorney, said. “Our city is tasked with protecting public safety for all people who call Seattle home. We will also protect our residents from unjust law-enforcement actions.”

"Unjust law enforcement actions"? Unbelievable.

Oakland authorities were unapologetic about defying the government:

Justin Berton, a spokesman for Oakland Mayor Libby Schaaf, said the city attorney, Barbara J. Parker, is reviewing the letter. Parker issued a statement saying “Oakland is proud to be a sanctuary city and is in compliance with federal immigration law, KRON 4 Bay Area reported. She said the city will respond to the letter “at the appropriate time.” “Our sanctuary city policy encourages the community to work with police and helps law enforcement to solve and prevent crimes in our city,” Parker said in the statement. “We will continue to focus our resources on fighting crime, rather than tearing apart Oakland families and making our city less safe.”

It's an uphill battle for the feds given the extremely broad interpretation of the law the courts have chosen to rule on. But making illegal aliens comfortable enough that a few of them might cooperate with police is hardly reason enough throw out federal immigration law. It remains to seen if DoJ will be able to apply sufficient pressure to move some of these sanctuary cities toward full compliance with the law.