AT&T has been under investigation for a few months by the Federal Trade Commission (FTC), following reports of data throttling on unlimited data owners, grandfathered in on older contracts. Data throttling is the intentional slowing of Internet service by a service provider.

But the Federal Communications Commission (FCC) may soon take over the case after AT&T spokesperson Mark Siegel said in an email "It is the FCC, not the FTC that regulates network management practices."

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FCC Chairman Tom Wheeler has already sent an open letter to AT&T, expressing deep concerns about the reported data throttling, which AT&T could be fined millions of dollars for abusing.

The FTC claims 3.5 million customers have received unnecessary data throttling, mostly on grandfathered unlimited data contracts, something AT&T no longer offers.

AT&T argues that its throttling complies with the FCC's 'maximum bit-rate' program, allowing carriers to slow down speeds in certain areas for unlimited customers, to prevent them from overwhelming the network.

"We manage our network resources in a manner that is transparent and fully consistent with the FCC's net neutrality rules," said Siegel.

AT&T says that while only 5 percent of all customers have an unlimited plan, they consume 25 percent of the network usage, as the unlimited users tend to watch more video and play more games on the mobile, without fear of substantial charges.

The FCC has argued that due to unlimited owners not knowing about the data throttling before signing up, it is against the law to start throttling data now.

AT&T is not the only U.S. carrier potentially in trouble for throttling unlimited data customers, Verizon Wireless has also had its fair share of complaints the FCC might like to follow up on.

TagsFCC, AT&T, Data throttling, unlimited mobile data