 -- The Federal Communications Commission hit AT&T with a $100 million fine today over allegations the telecommunications company slowed down data speeds for some customers who signed up for unlimited plans.

The agency alleges some AT&T subscribers with unlimited data plans had their speeds throttled after using a certain amount of data within a billing cycle. An AT&T spokesperson told ABC News the company "will vigorously dispute the FCC's assertions."

"The FCC has specifically identified this practice as a legitimate and reasonable way to manage network resources for the benefit of all customers, and has known for years that all of the major carriers use it," an AT&T spokesperson told ABC News. "We have been fully transparent with our customers, providing notice in multiple ways and going well beyond the FCC's disclosure requirements."

At issue is whether AT&T misled customers by labeling the plans as unlimited, which the FCC said violates a transparency rule in its 2010 Open Internet Order. The agency also alleged AT&T failed to sufficiently inform subscribers of the maximum speed they would receive.

FCC Chairman Tom Wheeler said "consumers deserve to get what they pay for."

"Broadband providers must be upfront and transparent about the services they provide," he said in a statement. "The FCC will not stand idly by while consumers are deceived by misleading marketing materials and insufficient disclosure."

An AT&T spokesperson said the FCC was aware of a network management policy of providing "more bandwidth to subscribers that have used the network less over some preceding period of time than to heavier users" as a solution to reducing congestion.