The Federal Communications Commission (F.C.C.) said on Thursday it was exploring whether new services from Comcast, T-Mobile USA and and AT&T violate net neutrality rules. In letters sent on Wednesday, senior F.C.C. officials asked the companies to provide information about promotions that allow unlimited streaming of video from specific content providers. Some legal experts have argued that the promotion unfairly prioritizes websites in a way that could hurt competition.

"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 incentivizing innovation and investment from all sources," the F.C.C's letter reads.

Tom Wheeler, the chairman of the F.C.C., said the letters did not signal a formal investigation of the companies, nor were the letters sent as part of an enforcement action.

"This is to help us stay informed as to what the practices are," he said.

But the letters asked the companies to provide details on technical and business aspects of their various plans.