More than 200 people were arrested on immigration violations during a four-day operation in Northern California, but authorities said today that hundreds eluded capture because of a warning from Oakland’s mayor.

U.S. Immigration and Customs Enforcement said officers made 232 arrests from Sunday to Wednesday arrests and renewed threats of a bigger street presence in California, where state law sharply limits cooperation with immigration authorities at local jails.

The Trump administration has cracked down on so-called sanctuary policies, insisting that local law enforcement inform federal agents when they are about to release immigrants discovered to be living in the country illegally.

Defenders of so-called “sanctuary” practices say they improve public safety by promoting trust among law enforcement and immigrant communities and reserving scarce police resources for other, more urgent crime-fighting needs.

Mayor Libby Schaaf tweeted on Saturday that an immigration operation was imminent in the San Francisco area, including Oakland, possibly within 24 hours.

It was unclear how many people would have eluded capture without the mayor’s warning. Targets often elude authorities because agents don’t have search warrants and advocacy groups have waged public awareness campaigns urging people not to open their doors. Other times, agents have outdated addresses or targets are not home.

The federal agency’s acting director, Thomas Homan, said Wednesday that Schaaf’s warning caused about 800 “criminals” to elude capture, an extraordinarily high number of missed targets.

More than 100 of the people arrested had criminal records, including convictions for child sex crimes, weapons charges and assault, ICE said. The agency didn’t release their names, except for one who was considered a high-profile target, making it impossible to verify individual cases.

“ICE has no choice but to continue to conduct at-large arrests in local neighborhoods and at worksites, which will inevitably result in additional collateral arrests, instead of focusing on arrests at jails and prisons where transfers are safer for ICE officers and the community,” the agency said.

White House spokesman Sarah Huckabee Sanders criticized Schaaf’s tweet.

“I think it’s outrageous that a mayor would circumvent federal authorities and certainly put them in danger by making a move such as that,” Huckabee said today. “And that’s currently under review by the Department of Justice.”

Sanders didn’t elaborate but Homan said Wednesday that the Justice Department was looking into whether the mayor obstructed justice.

The mayor has repeatedly defended her actions, saying Wednesday that she was not tipped off by “official sources” and that she didn’t reveal specific locations.

“It is Oakland’s legal right to be a sanctuary city and we have not broken any laws,” she said. “We believe our community is safer when families stay together.”