Alan Gomez, USA TODAY

Donald Trump vowed throughout his presidential campaign to punish "sanctuary cities" that fail to cooperate with federal immigration enforcement authorities. Yet three months into his presidency, the Trump administration still can't answer a simple question: What exactly is a sanctuary city?

That's a multimillion-dollar question.

The reason: The administration wants to withhold federal money from cities, counties and states that it considers "sanctuaries" for undocumented immigrants. The Department of Homeland Security has tried to publicly shame such jurisdictions, and the Department of Justice issued letters to nine local governments warning that they may lose federal grants if they are deemed sanctuaries.

Even so, when a group of mayors met this week with Attorney General Jeff Sessions and his staff in Washington and asked for a definition of the term, they got scant satisfaction.

"They told us, 'We haven't fully fleshed that out yet,'" said New Orleans Mayor Mitch Landrieu, who received one of the threatening Justice Department letters. "We're happy to work with the president, but they have to understand what they're talking about. You can't accuse us of violating the rules if you haven't told us what the rules are."

Even U.S. District Judge William Orrick, who on Tuesday struck down Trump's executive order to withhold all federal grants from "sanctuary cities," encouraged the administration to develop regulations or guidance on "designating a jurisdiction as a 'sanctuary jurisdiction.'"

When asked for a definition of a sanctuary city Wednesday, the Justice Department said it could not comment “in light of the pending litigation” before Orrick.

At stake are hundreds of millions of dollars in Department of Justice grants that help pay for local police, prosecutors, judges and jailers. Orrick ruled that only those kinds of grants can be withheld from local jurisdictions.

The local governments say they are being coerced into assisting with immigration enforcement — a federal responsibility — and engage in the legally questionable practice of detaining suspects solely to give Immigration and Customs Enforcement (ICE) agents time to investigate their immigration status.

The local leaders say that has caused unnecessary confusion for their budgets, since so many rely on a variety of federal grants. Santa Clara County (Calif.) Counsel James Williams said the vague, undefined threats from the Trump administration have been "hanging over our heads."

Read more:

Federal judge blocks Trump plan to punish ‘sanctuary cities’

Justice Department warns 9 'sanctuary' jurisdictions they may lose funding

Errors prompt Trump to halt reports shaming 'sanctuary cities'

"Sanctuary city" is not a legal term but a general term often used to describe more than 300 jurisdictions that don't fully comply with federal immigration efforts. Several administration officials have turned to a federal law when asked to define it. The law forbids any local agency from enacting a policy that orders its employees to withhold the immigration status of people in its custody.

"That means, according to Congress, a city that prohibits its officials from providing information to federal immigration authorities — a sanctuary city — is violating the law," read a White House statement Tuesday.

That doesn't explain how the Alachua County (Fla.) Sheriff's Office ended up on a report issued by Homeland Security last month shaming sanctuary cities. That department has a policy that forbids its agents from honoring federal requests to hold suspects for up to 48 hours for immigration agents. Sheriff's spokesman Art Forgey said courts have ruled that practice to be a violation of the constitutional due process rights of suspects, so they refuse to comply.

But Forgey said his office immediately shares information on every inmate that enters its jails with the local office of the federal immigration enforcement agency. He said deputies pass along the name, fingerprints, country of birth, criminal charges and even the estimated release date of each person in its jails. That's why it came as a shock to his office when it was listed as a sanctuary city.

"Where they come up with their information and how they come about it, it's perplexing," Forgey said. "The people at Homeland Security, I think they're ill-equipped to deal with the orders that have been placed upon them."