President Donald Trump’s administration sued Philadelphia and some of the city’s top officials Thursday over a new city ordinance that prohibits law enforcement officers from concealing their identities and effectively bans federal immigration agents from wearing masks.

The law, part of City Council’s recently adopted “ICE Out” package of legislation imposing some of the nation’s toughest local restrictions on immigration agents, is “blatantly unconstitutional,” the lawsuit said.

“Such an ordinance also undermines the principles of federalism that underlie our entire constitutional order by seeking to prevent effective federal law enforcement within Philadelphia,” according to the complaint.

The ordinance makes it a crime for any law enforcement officer, including Immigration and Customs Enforcement agents, to wear face coverings or conceal personal identifiers while carrying out their official duties in the city, and it requires officers to identify themselves. It also prohibits the use of unmarked vehicles.

An officer that violates this ordinance can be prosecuted, and risks up to 90 days in jail plus a fine.

The suit, filed in the Eastern District of Pennsylvania, names as defendants the city, Mayor Cherelle L. Parker, District Attorney Larry Krasner, and City Solicitor Renee Garcia. It asks a federal judge to find the bills unconstitutional, warning that federal agents could suffer irreparable harm if the policy remains in place.

“Protecting officers’ personal identities is particularly important during high-risk enforcement operations involving individuals with violent criminal history, gang affiliations, transnational criminal organizations, and known or suspected terrorists,” the suit says.

The lawsuit marks the Trump administration’s most significant action targeting Philadelphia’s immigrant-friendly policies to date.

Although Philly has long been known as a “sanctuary city,” Parker has largely avoided direct confrontation with the White House over the issue, a reversal from former Mayor Jim Kenney’s combative stance that has been credited by Parker’s supporters with keeping the city out ion Trump’s crosshairs.

Council members, however, wanted to take a more proactive stance against Trump’s nationwide deportation campaign. And they seem to have gotten his attention.

Councilmembers Rue Landau and Kendra Brooks, who authored the “ICE Out” package, did not immediately respond to a request for comment.

The Parker administration declined to comment.

In addition to banning officers from concealing their identities, the “ICE Out” package, which passed Council with a veto-proof supermajority in April, prohibits federal immigration agencies from staging raids on city-owned property, bans discrimination on the basis of citizenship status, and prohibits the city from engaging in most forms of information sharing with ICE.

The legislation also codified some of Philadelphia’s long-standing “sanctuary city” policies that were previously established only through executive order — most notably a ban on city jails honoring ICE detainer requests without judicial warrants.

Parker signed six out of seven bills in May, and allowed the ban on agents hiding their identity to become law without her signature.

Parker didn’t sign the bill after Garcia expressed concern about the ban’s “significant legal and operational challenges,” the suit notes. The mayor’s signature would signal the Parker administration’s intent to enforce the requirement, the solicitor said, and would send an inaccurate signal to the public that the prohibition is enforceable.

While Parker might have attempted to distance herself from the requirement by not signing the bill, the lawsuit quotes Krasner threatening federal agents with prosecution.

“We will arrest you. We will put handcuffs on you. We will close those cuffs. We will put you in a cell,” Krasner said in January, before the passage of the ICE Out bills. “We will do everything in our power to convict you and we will make sure you serve your entire sentence because Donald Trump has no power whatsoever to pardon you.”

The complaint makes clear that by bringing this lawsuit, the Department of Justice isn’t closing the door on challenges to other ICE Out ordinances. But it is notable that the Trump administration started with the mask ban.

The Ninth Circuit Court of Appeals upheld in April a lower court’s injunction to a California law that required federal agents to “visibly display identification.” The unanimous three judge panel ruled that the requirement violates the Constitution’s Supremacy Clause that bars the states from regulating federal government activities.

The Trump administration filed a similar lawsuit against New Jersey in April after Mikie Sherrill signed a mask ban into law. The litigation is ongoing.