Immigration and Customs Enforcement (ICE) conducted “Operation Safe City” this week, with nationwide immigration sweeps that led to the apprehension of more than 450 undocumented immigrants, many of whom were not originally targeted by the raids.

The enforcement action was intended to target so-called sanctuary cities—a term for jurisdictions that do not always honor detainer requests—according to the federal immigration agency.

A detainer is a key tool used by ICE to apprehend undocumented people who come in contact with law enforcement, according to the American Civil Liberties Union (ACLU). Made in writing, the request asks “that a local jail or other law enforcement agency detain an individual for an additional 48 hours (excluding weekends and holidays) after his or her release date in order to provide ICE agents extra time to decide whether to take the individual into federal custody for removal purposes,” the ACLU explains.

Detainer requests are not mandatory or legally required; local and state law enforcement agencies have no obligation to honor them. A detainer does not indicate probable cause and it does not operate as an arrest warrant.

As ICE officials often say, this week’s operation reportedly prioritized “criminal aliens”—people “with criminal convictions, pending criminal charges, and known gang members and affiliates.” But the agency’s numbers suggest otherwise.

Of the 315 undocumented immigrants with “criminal convictions,” the highest percentage—86 people—were targeted for deportation because they were charged with driving under the influence, a misdemeanor in all 50 states. Of the 498 people taken into custody, 183 were what ICE considers “collateral arrests” of undocumented immigrants in the area of the targeted enforcement area. 

Philip Miller, deputy executive associate director of Enforcement and Removal Operations for ICE, spoke Monday at a Migration Policy Institute conference at Georgetown Law School, where he blamed sanctuary cities for collateral arrests. Miller said these collateral arrests will continue.

Miller said that when jurisdictions choose not to honor ICE’s detainer requests, the agency is “forced to conduct enforcement on the streets.”

“People talk a lot about why do we do collateral arrests. You drive us to houses or apartments and for reasons of officer safety, if I go to your place and you allow me in, the officers there need to make sure the apartment is safe, so we need to interview everyone in there. Sometimes biometrically, sometimes biographically,” Miller said. “If we encounter someone who may not be the target of our enforcement operation, but is nevertheless in violation of the law, we have orders directing us to take those people into custody as well.”

“Ideally, if the community was working with us, if we were working in the jail, which would be our most desired outcome, those collateral arrests would not be taking place.”

Collateral arrests aren’t a new phenomenon, nor are they a result of cities claiming sanctuary status after Trump took office. Some cities, like Chicago, have had sanctuary policies in place for 30 years, though many offer little more than statements of solidarity to undocumented immigrants. Even in places like New York City, considered the most immigrant-friendly place in the country, jumping a subway turnstile can put an undocumented person in deportation proceedings. Many jurisdictions that claim sanctuary still honor ICE detainers, especially for serious crimes.

Miller’s assertion that collateral arrests happen primarily because cities do not honor ICE detainers is simply untrue. While director of ICE’s New Orleans field office beginning in 2009, Miller led the charge on collateral arrests. His arrests were so widespread in New Orleans, advocates called it “stop and frisk for Latinos.”

The Nation first reported in 2014 on ICE’s collateral arrests in New Orleans, which exploded under Miller’s watch. “Officially, all operations are pre-approved by field office leadership, and agents go into the field with a list of specified targets. But the spokesperson acknowledged that agents may go to locations where they have intelligence suggesting a target might be—and that they might then detain people who are ‘hanging around’ if the agents ‘discover’ that they have criminal records or have a reasonable suspicion that they do,” according to the Nation.

Collateral arrests were made possible through the Obama-era Criminal Alien Removal Initiative (CARI), which began in the spring of 2012. Though it was nationwide, in New Orleans it “morphed into an aggressive initiative characterized by coordination between ICE and local police, and the use of mobile fingerprinting devices wielded against seemingly random groups of Latino residents,” the Nation reported. Immigrants targeted by these raids reported being detained at “checkpoint-style operations” at apartment complexes, grocery stores, soccer fields, and laundromats.

“Agents rounded up whole groups of people at Bible study groups, soccer fields and other public spaces in Latino neighborhoods—and used the fingerprinting machines to figure out who had a criminal record,” the Nation reported. But people without criminal records were also detained, simply by virtue of being in the area and being undocumented.

ICE’s Operation Safe City came just weeks after the agency claimed to have cancelled Operation Mega due to weather conditions. The nationwide immigration sweep, which was planned to launch in mid-September and detain “historic” numbers of undocumented people, is being investigated by immigration advocates who don’t believe the operation has been cancelled.

This week’s immigration sweeps targeting so-called sanctuary cities are part of a larger effort under the Trump administration to retaliate against jurisdictions that do not always honor ICE detainers.

A March court proceeding revealed that ICE’s February immigration raids in Austin were conducted as retaliation against Texas’ Travis County Sheriff Sally Hernandez for limiting her jurisdiction’s cooperation with ICE, including refusing to honor ICE detainer requests to hold undocumented people for non-violent crimes.

As part of Hernandez’s policy, Juan Coronilla-Guerrero was one of the estimated 30 undocumented immigrants released from Travis County custody, despite having ICE detainers in their names. ICE then took unprecedented steps to detain him in March, sending agents in plainclothes to apprehend him at his court hearing.

At Monday’s conference, Miller said courthouse raids will continue, as they do not fall under the scope of ICE’s sensitive locations memo. Miller blamed the need to detain people at courthouses on sanctuary cities refusing to honor ICE detainers.

Guerrero has since been deported to his death.