Some NIPSCO customers recently opened their gas bills and saw charges much higher than normal.
The state is now investigating.
The Indiana Utility Regulatory Commission launched the review after NIPSCO reported problems while switching to new meters that track gas use remotely.
Some customers say their bills doubled or even tripled.
NIPSCO says the issue affects less than one percent of its gas customers and is reviewing accounts.
The company will refund up to a year of overcharges if it finds customers were billed incorrectly.
State leaders are still working to determine what caused the problem and when the investigation will conclude.
