He intends to acquire a stake worth between $184 million and $920 million, the Ohio-based company said, citing a letter from Icahn dated Feb. 16. (https://bit.ly/2NlMrh9)
The power utility said it does not know if the billionaire investor or his affiliates have already bought shares or derivatives of the company.
Icahn Capital did not immediately respond to a Reuters request for comment.
Last year, FirstEnergy had received subpoenas in a $60 million bribery case stemming from a controversial bill to bail out the state's nuclear power plants.
Ohio House Speaker Larry Householder and four other men tied to state politics were arrested in the case.
Shares of FirstEnergy closed up more than 7% on Thursday.
(Reporting by Niket Nishant in Bengaluru; Editing by Arun Koyyur)