California’s Supreme Court unanimously ordered Riverside County Sheriff Chad Bianco to return more than 650,000 ballots he seized during an investigation into alleged voting irregularities.

The court ruled that the seizure was unlawful because California law requires voted ballots to remain under the custody and control of election officials, even during criminal investigations.

Bianco had said he seized the ballots after receiving a complaint from a local group, although election officials had previously determined the allegations were unfounded.

California Attorney General Rob Bonta and voting rights advocates challenged the seizure, arguing that Bianco lacked authority over election materials and that the action could undermine public confidence in elections, NBC News has reported.

The ruling comes as officials and voting rights groups monitor similar efforts to obtain election materials, while California has enacted legislation specifically prohibiting the seizure of ballots.