The U.S. Justice Department announced it worked with local and state authorities to execute a series of arrests and seizures in an effort to take down what it called a national network of catalytic converter thieves and associates.

The sting, which took place in nine states across the country, resulted in the arrest of 21 people, and the Justice Department said it seized millions of dollars in assets, such as homes, bank accounts, cash and luxury vehicles.

Attorney General Merrick B. Garland said in a press release that “amidst a rise in catalytic converter thefts … we will continue to work alongside our state and local partners to disrupt criminal conspiracies like this one that target the American people.”

Catalytic converter thefts have surged as thieves have targeted the valuable precious metals contained in the component of vehicles’ exhaust systems intended to reduce pollutants. Some of the metals have more value per ounce than gold, the Justice Department said in its press release on the sting.

California has been a particular target of catalytic converter thieves due to its higher emissions standards, said U.S. Attorney Phillip Talbert of the Eastern District of California.

“Last year approximately 1,600 catalytic converters were reportedly stolen in California each month, and California accounts for 37% of all catalytic converter theft claims nationwide,” Talbert said in the press release. “I am proud to announce that we have indicted nine people who are at the core of catalytic theft in our community and nationwide.”

READ MORE: NADA Leads the Charge on Combatting Growing Problem of Catalytic Converter Theft