The Federal Communications Commission said Tuesday it is conducting an industry-wide investigation into cell phone billing practices amid complaints by customers of Verizon Wireless and other carriers of unwanted data charges.

The agency's 10-month-long investigation, which it previously said was focused on Verizon, could subject other carriers to penalties if the FCC finds those firms charged "mystery fees" that violate its "truth and billing" guidelines. The agency didn't disclose the names of other carriers that are being probed. An AT&T spokeswoman wrote in an e-mail that the company is unaware of an investigation into its practices. A Sprint Nextel spokesman said the company doesn't generally comment on investigative matters at the FCC.

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