The Federal Communications Commission has sent letters to AT&T, T-Mobile and Comcast seeking more information about free data plans. FCC Commissioner Tom Wheeler says the letters aren’t part of an investigation, but concerns have been expressed about whether the plans go against recently implemented net neutrality rules.

The FCC wants to have meetings before January 15.

Wheeler disclosed the letters at a news conference on Thursday.

The letters asked “them to come in and have a discussion with us about some of the innovative things they are doing,” Wheeler said, according to Bloomberg News.

AT&T is “reviewing the letter and will respond as appropriate. We remain committed to innovation without permission and hope the FCC is too,” an AT&T spokesperson said.

AT&T’s sponsored data option permits companies to pay customers’ wireless data charges while watching videos or viewing content on their phones and tablets, according to Bloomberg. AT&T has signed on at least six partners to the service.

Comcast noted that its “Stream TV service does not go over the public Internet. It is a cable service that only works in the customer’s home”

T-Mobile recently launched a “zero rated” service that doesn’t count video against data caps.

According to advocates of net neutrality, the concept that Internet access providers shouldn’t discriminate against some Web traffic in favor of others. They say innovation could be curtailed if smaller video providers couldn’t afford to pay for an exemption.

The exemptions come as mobile video booms. But video eats up a lot of data. On a smartphone, an hour or two of video consumes about 1 gigabyte. AT&T and Verizon, the biggest wireless companies, have been phasing out unlimited plans, so customers pay more to watch more over cellular networks.

Even home Internet providers are testing data caps. About 13 percent of Comcast’s customers are in a market with caps, according to research firm Moffett Nathanson. Comcast exempts its own video, but not video from rivals like Netflix. Critics say that could hurt competition, as consumers would be drawn to the service that’s exempt.

However, FCC Commissioner Ajit Pai told reporters said the letters would likely lead to an investigation. Reuters reported that Pai said the FCC shouldn’t “haul in companies left and right” to “justify their business plans,” he said. He called the FCC action a “fishing expedition.”

AT&T is among the carriers that are fighting the FCC net neutrality rules in court. A hearing took place in a federal appeals court earlier this month.

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