Officials in Portage, Chesterton and Valparaiso and Porter County said they have received calls about the smell.
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Chesterton officials said the sulfur-like smell is believed to have started in the vicinity of the oil refinery/steel mills in Lake County, Indiana.
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Michigan City officials said "due to a natural gas leak in Porter County, you may notice a heavy smell of gas outdoors throughout Michigan City."
Officials said there is no threat to public safety, and residents should try to keep the 911 dispatch lines open for active emergencies.
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The Northern Indiana Public Service Company, or NIPSCO, is investigating.
In a statement, NIPSCO said, "Throughout the afternoon and evening, we have received several hundred calls from customers regarding odors in the air. When we receive those calls, we send service crews and techs into those areas. Based on all the readings we've conducted, we have not sourced any of those calls to being related to natural gas. Calls have ranged from Porter, LaPorte, Lake and St. Joseph counties."
This is a developing story; check back with ABC7 Chicago for updates.