The U.S. Justice Department last week sent another round of letters to the so-called sanctuary cities of Seattle and Oakland, as well as the state of Vermont, demanding further proof that they are cooperating with federal immigration authorities.

The letters warn that the Justice Department could use subpoena power to force Seattle and Vermont to provide documents showing whether they are restricting information sharing between law enforcement agencies.

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."

"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." — U.S. Attorney General Jeff Sessions

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."

"The federal government does not get to run our cities or convert our local law enforcement officials into immigration cops." — Jenny Durkan, mayor of Seattle

In February, Durkan issued a directive requiring that all requests for city information or access by immigration authorities be routed through her office, the Seattle Times reported.

She further weighed in on the new pressure from the DOJ over Seattle’s immigration policies, the Times report said.

“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.”

Thursday’s DOJ letter requesting documents gives the city a May 14 deadline, the Seattle Times reported.

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.”

The Justice Department has threatened to deny grant money from communities that refuse to share such information.

However, a federal judge Wednesday sided with the city of Los Angeles and issued a nationwide injunction prohibiting the Justice Department from tying federal grants to local police departments’ cooperation with ICE, Axios reported.

The Justice Department also notified the District of Columbia and the Louisville-Jefferson County Metro Government in Kentucky that there is no evidence either jurisdiction is currently out of compliance with the federal statute.

The Associated Press contributed to this report.