Three members of a California family pleaded guilty Monday to conspiracy for their role in a ring that shipped $600 million worth of stolen catalytic converters from California to New Jersey, federal prosecutors said.

Brothers Tou Sue Vang, 32, and Andrew Vang, 28, along with their mother, Monica Moua, 58, were part of "a national network of thieves, dealers, and processors" who provided the stolen auto anti-smog devices to a metal refinery for more than $600 million, according to a statement from the U.S. attorney's office.

Twenty-one people from California and New Jersey have been charged in the case, prosecutors said. The three Sacramento family members pleaded guilty to conspiring to transport the devices in return for more than $38 million.

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Tou Sue Vang also pleaded guilty to 39 charges related to money laundering, prosecutors said.

Catalytic converters are easily stolen and contain precious metals including platinum.

"Some of these precious metals are more valuable per ounce than gold, and their value has been increasing in recent years," the U.S. attorney's office said. "The black-market price for catalytic converters can be above $1,000 each."

California accounts for 37% of catalytic converter theft claims nationwide, with about 1,600 reported stolen each month, federal prosecutors said.