Duke Energy's legal bill from North Carolina for the a 5-month state investigation last year came to a cool $3.5 million.

The N.C. Utilities Commission released the bill total on Monday in response to a records request. The last month billed was for November, but Utilities Commission General Counsel Sam Watson there could be a final bill with miscellaneous expenses.

The state investigation ended last month when Charlotte-based Duke settled the matter with the Utilities Commission, agreeing to changes in its executive ranks, including the mandatory departure of CEO Jim Rogers by Dec. 31, 2013.

Under state law, utilities finance the operations of the N.C. Utilities Commission and are also required to pay the costs of investigations. The bill submitted to Duke Energy only represents the Utilities Commission's cost of the probe, not Duke's cost of hiring multiple law firms to defend itself.