UPDATED: 2:40 p.m.

NORRISTOWN, Pa. (KYW Newsradio) — Delaware County Emergency Services officials are continuing to search for the cause of what is described as a petroleum or gasoline smell in the area.

County Director of Emergency Services Timothy Boyce said calls starting coming in around 10 a.m., reporting what he says is a quite strong smell.

Crews have been monitoring the air quality since then, and Boyce said it appears the odor is not hazardous or explosive.

"But until we can find the source," he said, "we’re going to keep at this.

"Our county's hazmat team is in the field along with state agencies, and we've contacted the numerous state partners we deal with to alert them to see if they know what the source is."

Boyce says they’ve reached out to the companies that run pipelines and refineries in the county, but no one is reporting any release, leak or accident.

"All of our refinery partners and our pipeline partners have resources in the field checking their facilities. They have not detected any leak but the best practice, we have people out in the field checking these locations," Boyce added.

In cooperation with the Coast Guard, he said they’re also checking waterways but haven’t found anything. They’re checking freight rail lines, too, to see if something leaked on the way through.

Initially the calls were coming from the southwest part of the county, but Boyce said it has spread into the central part as well. As of 1 p.m., they have received more than 25 calls.

"It’s growing. The initial reports were in the Concordville, Chester Heights area of the county. Now we have numerous reports in Aston Township, Brookhaven, Upper Providence. I would best say west of the Blue Route is where the reports are," he said.

Officials also checked with other counties, including in New Jersey and Delaware, and the smell appears to be centralized in Delaware County. He said the odor seems to be focused west of I-476 and north of I-95, but they’ve gotten calls as far north as Marple.

One school, the Chester Charter School at 1500 Highland Ave., is also closed, but Boyce called that an abundance of caution after the odor got in its ventilation system.

Officials are asking residents to call 911 to report the odors. For now, Boyce said if the odor is strong where you are, stay inside and shut down exterior ventilation systems.

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KYW Newsradio's Hadas Kuznits contributed to this report.