FirstEnergy said in a release it had received subpoenas in connection with the investigation, was reviewing details and intends to fully cooperate with the probe.
Republican Ohio Governor Mike DeWine called for Householder to resign immediately.
In recent years, FirstEnergy was among several companies that have lobbied state and federal officials to get subsidies to keep reactors in service.
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Aging nuclear plants have suffered from higher security costs and competition from power plants that burn cheaper natural gas.
FirstEnergy in early 2018 asked the US Department of Energy to intervene in markets by using emergency powers to direct grid operators to buy electricity from coal and nuclear plants.
US President Donald Trump directed then-Energy Secretary Rick Perry to use an independent office of the department's authority in a way that would help plants like those operated by FirstEnergy.
Federal energy regulators rejected those overtures, instead taking up a study of how to make the electrical grid more resilient.
The companies claimed the plants were needed to support state clean energy programs and without those reactors, several states would have had a hard time meeting their clean energy goals.
This month, Exelon Corp's unit ComEd agreed to pay $200 million to resolve a US Department of Justice probe over lobbying practices in Illinois.