"We can confirm reports of an FCC investigation into mystery fees that appeared on Verizon Wireless bills costing over 15 million Americans tens of millions of dollars," Michele Ellison, FCC enforcement bureau chief, said in a statement.

The customers were wrongly charged for data or Internet use and will receive a refund either in the form of credits of $2 to $6 on their bills or checks, Verizon said a statement Sunday.

The affected customers did not have data usage plans, which allow access to the Internet, and were charged because of exchanges initiated by software built into their phones, Verizon said.

The cost of the refunds could be as high as $90 million, the New York Times reported. The paper also reported that the FCC received complaints from Verizon customers about the wrong billing over a period of three years.

"Questions remain as to why it took Verizon two years to reimburse its customers and why greater disclosure and other corrective actions did not come much, much sooner," Ellison said.

"The Enforcement Bureau will continue to explore these issues, including the possibility of additional penalties, to ensure that all companies prioritize the interests of consumers when billing problems occur," she added.

Shares of Verizon were lower in pre-market trading Monday.