An immigrant from El Salvador being searched on the tarmac before boarding an MD-80 aircraft for a repatriation flight of 80 immigrants to their home country in 2012 at Phoenix-Mesa Gateway Airport in Mesa, Arizona. Associated Press/Matt York As the Trump administration ramps up deportations and cracks down on so-called sanctuary cities, mayors and prosecutors across the US say immigrants who are living in the country illegally have begun avoiding law enforcement at all costs — often declining to testify in court cases and refusing to report crimes.

In Denver, four women who recently made domestic-abuse allegations declined to pursue their cases out of fear they would be seen at the courthouse by Immigration and Customs Enforcement officers and deported, according to Denver's city attorney, Kristin Bronson.

Their reticence, Bronson said, came after a video was released last month showing ICE agents waiting in a Denver courthouse and telling an attorney they were there to make an arrest.

"Without victims willing to testify we've had to dismiss those charges and the violent offenders have seen no consequences for their violent acts," Bronson told NPR. "We have grave concerns here that they distrust the court system now and that we're not going to have continued cooperation of victims and witnesses."

An ICE spokesman told NPR that the agency's officers generally would make arrests in courthouses only if they had "exhausted other options."

Similar scenarios have played out in cities across the US. Prosecutors in Austin, Texas, said they encountered at least one recent instance of a woman who refused to cooperate with investigators in a domestic-violence case because she feared being deported. That incident came shortly after ICE officers arrested a man inside a courthouse where he had been scheduled for a routine hearing, the Austin American-Statesman reported.

Mayor Steve Adler of Austin mentioned the woman in a conference call with reporters Tuesday, saying the county's ability to prosecute criminals depended on victims' and witnesses' willingness to come forward.

"That safety we enjoy in this community is due in part to the trust relationship between our public safety officers and the community," Adler said.

Austin is in the heart of Travis County, Texas, which has already come under fire from the Trump administration for recently announcing its intention to refuse to honor most federal requests to detain those suspected of violating immigration laws past their scheduled jail release dates for unrelated charges.

The Department of Homeland Security released a report last week calling out jurisdictions that had refused to honor so-called detainer requests. Travis County accounted for 142 of the 206 instances listed.

Police crime tape displayed at the scene where a 16-year-old boy was fatally shot in the head and another 18-year-old man was wounded by gunfire on the 7300 block of South Sangamon Street on April 25 in Chicago. Getty Images/Joshua Lott

'We have to build relationships'

Some police officials have argued that cracking down on immigration actually harms public safety. A 2013 study has bolstered that argument, finding that up to 70% of unauthorized immigrants said they would be less likely to call the police for help because they feared being deported.

Chicago is in a particular predicament because of both its status as a sanctuary city, or city whose police force tends not to comply with federal immigration requests, and its soaring murder rate. The city has been a target of public scorn from Trump, who has accused the mayor there of being soft on crime and who previously threatened to "send in the feds" to solve the city's gun-violence crisis.

Yet officials in Chicago have maintained that they won't use the city's police department to enforce federal immigration law.

"In local departments we have to build relationships, and we build it on trust," retired Chicago police officer Richard Wooten told Chicago magazine. "It's one of the major components we use to solve crimes in the community. We'll never have that kind of relationship if we begin to enforce immigration."

In other cities, officials have said they fear that trust has already been compromised, with unauthorized immigrants not even reporting perpetrators to the police, let alone testifying against them in court.

In Texas, the county attorney for El Paso County, Jo Anne Bernal, told The Guardian that three victims sought to withdraw their cases because of immigration fears.

Bernal reported an "alarming" 12% drop in people seeking protective orders, shortly after a woman living in the US illegally was arrested at a local courthouse where she was seeking a protective order. Bernal said she couldn't be certain the drop was due to unauthorized immigrants but told The Guardian the numbers were unusual.

The Los Angeles Police Department on Tuesday announced that the number of sexual-assault reports from the city's Latino population this year had plunged 25% from the same period last year, and domestic-violence reports have dropped by 10%.

"Imagine, a young woman, imagine your daughter, your sister, your mother … not reporting a sexual assault, because they are afraid that their family will be torn apart," the Los Angeles police chief, Charlie Beck, told the Los Angeles Times. He said reports of sexual assault and domestic violence had not dropped among other ethnic groups in the city.

ICE, however, dismissed the LAPD's numbers and said its officials considered whether individuals were victims of crimes before deciding to deport them.

"The greater threat to public safety is local law enforcement's continuing unwillingness to honor immigration detainers," ICE spokeswoman Virginia Kice told the Los Angeles Times.

"Rather than transferring convicted criminal aliens to ICE custody as requested, agencies, including the Los Angeles Police Department, are routinely releasing these offenders back onto the street to potentially reoffend, and their victims are often other members of the immigrant community."