Judge Andre Birotte Jr. ruled Thursday in U.S. District Court that immigrants were held too long, denied bail or were locked up despite a lack of evidence.
The ruling came in a pair of class-action cases that were combined because they involve the use of immigration detainers issued by the federal government to hold suspected illegal immigrants up to 48 hours after they are to be released from jail so immigration agents can pick them up for deportation.
"It is an important precedent for the rest of the country where the law enforcement agencies are still honoring detainers," said attorney Jennie Pasquarella of the American Civil Liberties Union of Southern California.
Birotte found the Sheriff's Department violated the detainees' Fourth Amendment rights against illegal search and seizures, which also protects against arbitrary arrests.
"Officers did not have probable cause that the individuals were involved in criminal activity, but were instead holding these individuals on the basis of civil immigration detainers," Birotte wrote. "Officers have no authority to arrest individuals for civil immigration offenses."
In a couple instances, the suspects being held were U.S. citizens.
California has since passed laws barring local law enforcement from honoring detainer requests, but the ruling affects thousands of people who were held by the nation's largest Sheriff's Department between 2010 and 2014 and could be eligible for monetary damages.
Los Angeles County, and many other California counties, stopped honoring immigration detainers after a federal court ruling in Oregon on the issue.
"This is another court joining the chorus and saying, these things are illegal, and this is the first time it happened on a class-wide basis," Pasquarella said.
The court also found that the Sheriff's Department was unfairly holding immigrants who should have been eligible for low bail amounts because they had detainers.
A sheriff's spokeswoman said the lawsuit filed in 2012 involved past practices.
"The Sheriff's Department does not detain any inmate beyond their normal release date regardless of whether or not there is a valid ICE detainer," said Nicole Nishida. "This has been the standard practice since 2014."
In the case against U.S. Immigration and Customs Enforcement, Birotte found that federal agents couldn't issue a detainer for an arrestee solely because the person was born in another country and lacked an immigration file in federal databases.
An ICE facility in Southern California issued detainers for that reason, which could affect U.S. citizens born overseas and people whose cases had typographical or other errors, in addition to foreigners who entered the country illegally, Pasquarella said.
Immigrant advocates had been seeking a ruling that would find all detainers unconstitutional that were based on database searches because they are unreliable, said Jessica Bansal, litigation director at the National Day Laborer Organizing Network. Birotte said that issue would need to be resolved at trial.
One witness in the case for ICE testified that about a third of database searches returned incorrect information.
"That wasn't the only problem with the databases, but that's the one that their witness offered up," Bansal said. "I wouldn't use a database that is wrong three out of 10 times to throw someone in jail."
Other issues that need to be resolved at a trial or further hearings include cases of immigrants subject to higher bail amounts and the basis for other detainers issued by the federal immigration agency.
A spokeswoman for U.S. Immigration and Customs Enforcement said the agency doesn't comment on pending litigation.
Taxin reported from Santa Ana, California. AP writer Michael Balsamo contributed to this report.
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