The Federal Communications Commission wrote to AT&T and T-Mobile US asking for more information about their packages that offer free data to users, also known as zero rating.

In addition to the two mobile operators, the regulator also wrote to cable operator Comcast about its live streaming service delivered over a fixed network.

FCC chairman Tom Wheeler said the letters asked the three operators “to come in and have a discussion with us about some of the innovative things that they are doing.”

However, Wheeler emphasised the FCC is not conducting an investigation but more of a fact-finding mission.

He recently give the all clear on net neutrality grounds to T-Mobile’s Binge On service, which waives data charges for certain video streaming services, although with the caveat he would keep his eye on it.

Binge On was only launched recently but AT&T has offered sponsored data for longer. Its packages allow content providers to subsidise users’ wireless data. This means users can stream specific content from sponsors without it coming out of their data allowance.  AT&T is thought to have signed up at least six partners for its sponsored data service.

The letter to AT&T has the air of the FCC looking for more information rather than a formal inquiry.

“As you may be aware, concerns have been expressed about these programs, for example, some have argued that sponsored data unfairly advantages incumbent content providers,” the letter to AT&T said, quoted by Reuters.

“We want to ensure that we have all the facts to understand how these services relate to the commission’s goal of maintaining a free and open Internet while incentivising innovation and investment from all sources,” it adds. The operators have until 15 January to respond.