Pedestrians walk by an AT&T store on October 23, 2013, in San Francisco. (Photo by Justin Sullivan/Getty Images) The FCC is planning to make AT&T pay $100 million for "misleading" customers about their unlimited data plans, it announced Wednesday.

AT&T slowed down the data speeds for customers who were on the unlimited plan, but never told them they would be receiving slower-than-normal service, the FCC's investigation reportedly found. It's what's known as "throttling" a connection.

The FCC says it received thousands of complaints after AT&T started capping data in 2011, and the company's throttling practices affected millions of people. The connection was allegedly slowed an average of 12 days per billing cycle, "significantly impeding their ability to use common data applications such as GPS mapping or streaming video."

"Unlimited means unlimited,” said FCC Enforcement Bureau Chief Travis LeBlanc in the release.

AT&T spokesperson Michael Balmoris said it plans to "vigorously dispute" the decision.

"The FCC has specifically identified this practice as a legitimate and reasonable way to manage network resources for the benefit of all customers, and has known for years that all of the major carriers use it," Balmoris said in an emailed statement. "We have been fully transparent with our customers, providing notice in multiple ways and going well beyond the FCC’s disclosure requirements.”

AT&T is already facing a similar suit from the Federal Trade Commission over its throttling practices. The FTC's October 2014 investigation found that AT&T allegedly charged customers for unlimited data plans, but then slowed speeds down, sometimes up to 90%. The FTC estimated that this affected 3.5 million customers.