(CN) - Congresswoman Anna Luna told the mayors of Boston, Chicago, Denver and New York City on Wednesday that she would be criminally referring them for investigation to the Department of Justice.

The Republican Florida representative said she believed all four cities were in violation of a federal law that prohibits "concealing or harboring unauthorized aliens."

"As soon as I leave here, these will be going over to Pam Bondi," Luna told the mayors, holding up copies of the referrals and referring to the U.S. attorney general.

Courthouse News requested copies of the referrals from Luna's office, but did not receive a response.

Luna publicly announced her intent to make the criminal referrals during the GOP-majority House Oversight Committee's hearing on sanctuary city policies.

Committee chair and Republican Kentucky representative James Comer announced last month that the mayors - Michelle Wu of Boston, Brandon Johnson of Chicago, Mike Johnston of Denver and Eric Adams of New York City - would testify at the hearing.

There is no set definition for what constitutes a "sanctuary city," though Republicans used the term Wednesday to refer to cities that limit local collaboration with federal immigration authorities.

Chicago's Welcoming City Ordinance, for example, establishes that civil immigration enforcement is the federal government's responsibility. It bars Chicago's city agencies from cooperating with U.S. Immigration and Customs Enforcement on civil immigration enforcement actions, though it allows for cooperation with ICE on some matters related to criminal proceedings.

The ordinance further prohibits city agencies from requesting or disclosing peoples' citizenship or immigration status, "unless such inquiry or investigation is required by Illinois State Statute, federal regulation, or court decision."

Johnson and the other mayors defended their cities' policies as important for maintaining public safety and immigrants' trust of public agencies.

"Put simply, any actions which amplify fears of deportation make Chicago more dangerous," Johnson said. "Those fears cause witnesses and victims to avoid cooperating with police."

Wu echoed his remarks.

"A land ruled by fear is not the land of the free," the Boston mayor said, after detailing anxious conversations she said she had with Boston residents.

"I spoke with pastors whose pews are half-empty on Sundays, doctors whose patients are missing appointments, teachers whose students aren't coming to class, neighbors afraid to report crimes in their communities and victims of crime who won't call the police," she said.

Several Republican members of Congress at the Wednesday hearing nevertheless claimed policies like Chicago's violated federal law, including the supremacy clause of the U.S. Constitution.

The supremacy clause establishes federal law's precedent over the laws of states or municipalities. Luna was only one of multiple Republicans at the hearing who suggested the mayors should face criminal charges.

"You gave sanctuary to people who are here illegally, you violated your oath to uphold the constitution," GOP Alabama representative Gary Palmer told Johnston at one point.

Anti-immigrant sentiment has been a major element of President Donald Trump's political project since his first term. The Chicago area in particular has been a target of the president and other Republicans' ire.

Donald Trump's "border czar" Tom Homan told Illinois Republicans in December that Chicago would be "ground zero" for mass deportations; last month the Trump administration sued the city, Cook County and the state of Illinois in federal court for violations of the supremacy clause.

Source: Courthouse News Service