One morning last month, Woodacre resident Tim Cain started his 2009 Toyota Prius, and what he heard sounded more like a Harley Davidson motorcycle. A loud revving noise was coming from under the passenger’s seat, so he laid on the ground and found a large cavity that wasn’t supposed to be there. His catalytic converter was stolen—one of more than 350 thefts in Marin this year. A couple hours later, he was in his garden and heard the same sound—his neighbor’s Prius had been hit, too. Catalytic converter thefts are on the rise in Marin and across the nation, and law enforcement officers are struggling to catch the culprits. The emissions-control devices contain precious metals like platinum, palladium, rhodium and gold that are used in chemical reactions to convert byproducts of combustion into less toxic substances. As the prices of these metals have gone up in recent years, so have thefts. Replacing a catalytic converter costs thousands of dollars; thieves can also damage a car’s heat shield and exhaust manifold when they’re sawing the part off. Most of the thefts have occurred in cities: San Rafael has seen at least 124 thefts, while the Marin County Sheriff’s Office has taken 62 reports in unincorporated areas. The latest victim parked their Prius on Mesa Road in Point Reyes Station last weekend. Generation 2 Prius models, from 2004 to 2009, are the number-one target because the cars are light, and their devices are easy to remove. Mr. Cain’s neighbor has a 2010 Prius that was left untouched next to the 2008 Prius that was robbed. Brenton Schneider, a spokesman for the sheriff’s office, suggested that people etch their license plate number into their catalytic converter, because the parts aren’t serialized. This will increase law enforcement’s ability to prosecute these crimes, locate stolen property and tie anyone found with them to a specific vehicle, crime and victim. He also recommended parking in a garage or well-lit area. Shields can be purchased for around $200. “These crimes are incredibly hard to investigate, unless someone is caught in the act,” Sgt. Schneider said. In March, a citizen reported someone cutting up exhaust parts in a storage unit in Novato. Deputies discovered a couple on the scene with several hundred parts, along with evidence that indicated that many more converters had been previously disassembled there. Over 300 catalytic converters were recovered from the site, with more than 60 from Priuses. Receipts showed the couple had sold over 2,500 catalytic converters since January to a recycler in Southern California, receiving several hundred thousand dollars in exchange. The couple was arrested, one of the few successful catalytic converter theft busts.