SACRAMENTO (CBS13) — Suspects have stolen more than 100 storm drain grates in Sacramento in recent weeks, forcing city workers to rush to replace them and leaving residents at a loss to explain by the culprits’ motives.

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Natomas residents said at least 13 of the cast iron grates went missing overnight Wednesday, and city workers have begun welding the replacement grates in place.

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The covers cost about $63 each to replace, but it is unlikely that thieves snatched the metal to sell for scrap. Although prices for recycled copper have jumped to about $3.25 a pound, cast iron can only sell for about a penny if you can find a recycler willing to pay for it.

At least one cover disappears a night, according to city officials, but suspects take as many as 15 in other nights.

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The city of Sacramento has already paid at least $5,300 in materials alone.