FCC Chairman Tom Wheeler. (AP Photo/Pablo Martinez Monsivais)

Federal regulators have asked AT&T, Comcast and T-Mobile to meet with them to discuss a growing and controversial business tactic: Offering Americans access to certain online services, such as Netflix or Spotify, without those services eating away at their monthly data caps.

Tom Wheeler, the chairman of the Federal Communications Commission, told reporters Thursday that his office has sent letters to the companies to "make sure we are informed as to what is going on."

He added quickly that the meetings are not a prelude to an investigation or legal action by the FCC.

"Let me be real clear: These were 'let's get informed' " letters, Wheeler said. The agency also said Thursday that it would be inviting others, such as consumer groups and industry associations, to meet with officials as well. Here are the letters for AT&T, Comcast and T-Mobile, which lay out a deadline of January 15.

The corporate practice of exempting partner websites or services from data caps — known in many cases as "zero-rating" — has the potential to help consumers get more use out of their monthly data plans. But critics have alleged that such arrangements can put smaller businesses at a disadvantage, particularly if they cost them money, and could tilt the online marketplace toward larger, more powerful companies.

Determining which side is right is up to the FCC, whose recent net neutrality rules empower it to examine carriers' practices on a case-by-case basis. AT&T and T-Mobile have long experimented with zero-rating. Verizon said this month that it would soon begin a limited trial of sponsored data, which could roll out more widely next year. Comcast has said that its own, proprietary streaming TV service — appropriately named Stream TV — will not count against consumers' data caps if they have one.

Comcast said Thursday that it is "happy to cooperate" with the FCC's fact-finding mission.

"We are reviewing the letter and will respond as appropriate," said Michael Balmoris, a spokesman for AT&T. "We remain committed to innovation without permission and hope the FCC is, too."

T-Mobile said it is looking forward to talking to the FCC about its zero-rating program.

"We believe it is absolutely in line with net neutrality rules," said Tim O'Regan, a company spokesman.