Experian North America chief executive Craig Boundy said in his own statement, "We sincerely apologise for the concern and stress that this event may cause."
It was unclear when the breach was discovered, but Experian said the matter was reported to the authorities immediately after it learned of the hack.
The company said in a statement, external: "We continue to investigate the theft, closely monitor our systems, and work with domestic and international law enforcement. Investigation of the incident is ongoing.
"Experian is notifying the individuals who may have been affected and is offering free credit monitoring and identity resolution services for two years. In addition, government agencies are being notified as required by law."
The firm said there "is no evidence that the data has been used inappropriately".
There have been a string of high-profile hacks of businesses and other organisations in recent years affecting millions of people, including adultery website Ashley Madison, Sony Pictures, and retailers such as Home Depot, Target, and eBay.
T-Mobile is now the third biggest mobile firm in the US, having surpassed Sprint this year.