The trial probably won't last longer than a few days. But much is at stake, for Philadelphia and potentially for other "sanctuary cities" around the United States.

On Monday in federal court in Center City, the trial began in the city's lawsuit against the Trump administration, over U.S. Attorney General Jeff Sessions' decision to withhold federal money from Philadelphia because of its sanctuary status.

At issue is whether the Trump administration can legally hold those funds unless the city government makes its police and law enforcement officers actively assist federal immigration agents in holding and deporting undocumented immigrants.

City officials say law-abiding, undocumented victims and witnesses of crimes won't come forward if they fear they could be deported.

"At our very foundation, we have to have the trust of our communities," First Deputy Managing Director Brian Abernathy testified Monday in U.S. District Court. "People are scared. We believe crimes are going under-reported. … People have to know they can depend on the Police Department and depend on us."

The money at stake is relatively small — a $1.5 million federal grant to a city with a $4.4 billion budget — but the principle is big. The Trump administration argues that sanctuary city policies allow dangerous criminals to be released when they should instead be deported to their homelands.

A key issue is the city's refusal to honor detainers from U.S. Immigration and Customs Enforcement, known as ICE. The city says it will honor those requests to hold on to people in custody only if accompanied by a signed warrant from a judge.

Under questioning from Justice Department lawyer Daniel Schwei, Abernathy confirmed that in 2015 the city had in custody an undocumented Honduran man, Ramon Aguirre-Ochoa, also known as Juan Ramon Vasquez. He was released when city officials did not honor an ICE detainer.

He was rearrested in 2016, for raping a child, and now is serving eight to 20 years in prison.

Abernathy said ICE could have obtained a judicial warrant — which the city always honors.

City officials reject the title of "sanctuary city," saying they merely enforce policies that provide equal treatment for people who come in contact with the criminal-justice system, regardless of immigration status.

Immigrants who commit crimes are arrested and charged, just like anyone else, city officials said.

In November, U.S. District Judge Michael Baylson issued an injunction that blocked the federal government's attempt to withhold the funds. He's now hearing the trial.