The Trump administration on Thursday filed a lawsuit challenging a Milwaukee city ordinance that prohibits law enforcement from wearing masks in most cases.

The lawsuit is the latest in a series of Justice Department challenges to state and local laws regulating masked federal immigration officers. Federal courts have blocked enforcement of similar laws in several jurisdictions, including Philadelphia, while litigation proceeds.

The ordinance was approved after Milwaukee Common Council members objected to Immigration and Customs Enforcement agents operating in the city with masks covering their faces.

The council voted in April to prohibit local and federal law enforcement officers from covering their faces while performing their duties. Officers who violate the rule face a penalty of $5,000 to $10,000.

The ordinance made exceptions for officers wearing face coverings to protect against extreme weather or while engaged in undercover operations.

"State and local leaders cannot tell Federal officers how to do their job," Associate Attorney General Stanley Woodward Jr. said in a news release. "And they certainly cannot prohibit them from ensuring their own safety in safeguarding our communities and enforcing federal law."

The lawsuit argues that federal law enforcement officers need flexibility in deciding when to identify themselves, and that Milwaukee's mask ban "would undermine officer safety and operational effectiveness."

The suit argues that federal officers are facing an increase in threats and harassment.

Some demonstrators take photographs of officers' faces, run them through facial recognition software, and disseminate their personal information online, according to the complaint. That could lead to assaults and doxxing.

"We will never tolerate the doxxing or harassment of federal officers, or any law like Milwaukee's that enables violent behavior against law enforcement officers and their families," Woodward said.

The mask ban did not become a flash point until city officials said they intended to enforce it after an ICE operation in late June that extended into July.

The Department of Justice sent the city a letter July 10 seeking assurances that the ordinance would not be enforced against federal officers.

In a July 17 response, city attorney Evan Goyke argued the ordinance does not conflict with federal law because no federal statute or regulation requires officers to wear masks or conceal their identities.

He wrote that "in the absence of any federal statute or regulation authorizing or requiring masking and identity concealment," the ordinance "only incidentally affects the mode of carrying out the officers' duties without preventing the federal government from enforcing federal immigration law."

"In closing, this office approved the Ordinance as legal and enforceable," Goyke wrote. "Neither the preliminary court rulings in other jurisdictions nor your letter convince us to change that opinion.

"We will not advise our clients to exempt federal law enforcement officers from the Ordinance and we will prosecute properly cited offenders."

DOJ said threatening officers with prosecution for "simply protecting their identities and their families" chills the enforcement of federal law and compromises sensitive law enforcement operations.

"Milwaukee's direct regulation of federal law enforcement operations threatens severe consequences for officers and public safety alike," assistant Attorney General Brett Shumate of DOJ's Civil Division said in the release. "The Federal Government, not the city, gets to decide which equipment federal agents may or may not wear when performing their official duties."