The FCC expressed concerns that zero-rating offers from both companies could e in violation of rules that prohibit carriers from favouring their own content by offering free data services.
AT&T launched a free data package that allows it customers to stream DirecTV over its network without using their monthly data allowance in September, but made the service free to all of its customers last week.
In a letter to the telco, which was published by Ars Technica, the FCC warned preliminary investigations into the service found AT&T’s practices “inhibit competition, harm consumers and interfere with… the benefits of the Open Internet.”
The regulator has now demanded AT&T provides further information before 15 December ahead of a final decision.
"These are incredibly popular free services available to millions of customers," AT&T said in a statement defending its offer. "Once again, we will provide the FCC with additional information on why the government should not take away a service that saves consumers money."
In a separate letter, the FCC warned Verizon that its FreeBee Data360 offer could also be in breach of net neutrality rules. The regulator’s investigation into Verizon is ongoing.
It is the latest in along line of disputes between the FCC and the US carriers, since the former introduced rules around net neutrality in 2015.
In June, a US court backed the FCC rules, which limit the amount telecoms companies can prioritise and manage traffic across their networks. President Barack Obama has also backed the FCC, but incoming president Donald Trump has previously expressed opposition to the rules, leaving their future in doubt.
