A mysterious stench once again struck parts of Delaware County, Penn., this week, overwhelming residents with its pungent odor and baffling investigators.

The smell has recently been reported in towns across the county, including in Glenolden, Tinicum and Upper Darby, just outside of Philadelphia. While no one knows what the elusive smell is or where it is coming from, residents have described it as fuel-like.

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“It’s like a sulfur, smokey,” Glenolden resident Terry Pelham told KYW-TV. “As soon as I got out of the car I could smell it.”

“It was overwhelming,” resident Diana Delaney said. “I was afraid to smoke. I was like, ‘Where is this coming from?’ I literally thought the gas main had broken.”

Another Glenolden resident, Michael Szymanski, speculated that it could be coming from the nearby creek or from sewage.

Delaware County residents have reported the smell wafting in and out of neighborhoods over the last several months. Last October, the smell shut down several businesses and a school in Chester before striking Brookhaven in December, FOX29 Philadelphia reported at the time.

The most recent stench is consistent with past reports, according to officials, but investigators still can’t find the source.

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Tim Boyce, director of emergency services for Delaware County, called it the “whodunit factor” when speaking to the Philadelphia Inquirer last month.

“The challenge we’re up against when you’re on the ground level is, the wind can be blowing in different directions, depending on where you are,” Boyce said. “By the time someone gets to where the report came from, it might not smell anymore.”

Investigators have been looking at local industrial facilities and pipelines in the area, but have yet to find a culprit, the paper reported at the time.

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The Odor Investigation Task Force, a collaboration of agencies from Pennsylvania, New Jersey and Delaware, is helping with the investigation.