An FBI agent investigating possible fraud obtained warrants from two Alameda County judges in 2016 to search a cell phone and use a tracking device, and said he found incriminating evidence.

The only problem, a federal judge said Tuesday, was that California law prohibits state and local courts from issuing warrants to federal officers — a fact that was apparently unknown to the agent, the FBI, and possibly even to the judges who approved the warrants.

The law authorizes state courts to issue search warrants to “peace officers,” who include police, sheriffs’ deputies and other state and local law enforcement officials, but not federal agents, said U.S. District Judge Vince Chhabria of San Francisco.

While some states give federal agents the same court access as local police, “California chose to limit federal authority to enforce state criminal laws,” Chhabria said. He ordered his ruling published in legal casebooks and distributed to the FBI, prosecutors’ offices and other agencies, “to put the relevant actors in the criminal justice system on notice.”

A judge’s ruling that a search was illegal may not prevent prosecutors from using evidence uncovered in the search as long as the officers acted in “good faith,” the standard set by the U.S. Supreme Court. But Chhabria said the searches in this case were rife with irregularities, including their reliance on evidence from two other searches that may have been illegal as well as information from a questionable informant. Finding “systematic and reckless misconduct,” he barred any use of the evidence.

John Jordan, a lawyer for defendant Donnell Artis, said the ruling should have “a big impact on the case,” to be determined at a hearing scheduled for July 17.

The U.S. attorney’s office in San Francisco was not immediately available for comment....