The Naperville-based company says it is facing possible civil fraud charges for allegedly filing false and misleading reports with the Securities and Exchange Commission.
The Illinois Commerce Commission is also looking into allegations that the company's Nicor Gas utility unit improperly accounted for gas purchases between 2000 and 2002.
Shareholders had accused company executives of inflating the financial results of a rate program. Nicor admitted no wrongdoing in the settlement which must still be approved by a federal judge.
Nicor Gas serves more than two million customers in Illinois, mostly in the Chicago suburbs and areas as far south as Bloomington.