Fracking began at a well in rural eastern Ohio last month. On Monday, parts of the surrounding Mahoning County started shaking, prompting state officials to shut down the operation, fearing it was responsible for what could be an unprecedented string of earthquakes linked to natural gas extraction.

Four earthquakes with magnitudes as high as 3 were felt Monday in Poland Township and in the village of Lowellville, sparking the immediate shutdown order. Another earthquake struck on Tuesday. Ohio oil and gas inspectors have been visiting the fracking site at the Carbon Limestone Landfill in Lowellville this week, trying to figure out whether it was responsible for the temblors.

“Out of an abundance of caution,” a state official said, “we notified the only oil and gas operator in the area and ordered them to halt all operations until further assessment can take place.”

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Links between earthquakes and the disposal of wastewater by frackers have been well established in recent years. The use of a single injection well, into which frackers were pumping their polluted wastewater at high pressure, was linked to 167 earthquakes around Youngstown, Ohio, in 2011 and 2012, prompting the state to put an end to its use.

If the recent string of Mahoning County earthquakes is found to have been caused directly by fracking, it would be the first such confirmed case.