The Federal Trade Commission has sued AT&T for promising unlimited data to wireless customers and then throttling their speeds by as much as 90 percent, the FTC announced Tuesday.

All major carriers throttle certain customers during times and places of congestion, as we've reported previously. AT&T seems to have earned the FTC's wrath by throttling customers regardless of whether they were trying to use their phones in congested areas, however. As we've also written, AT&T was throttling unlimited subscribers regardless of network conditions until July, when it changed its policy. Throttling was enforced once users hit 3GB or 5GB of data per month. AT&T still throttles customers but now says it only does so in congested areas. (UPDATE: It turns out AT&T is still throttling LTE regardless of network conditions.)

The FTC's lawsuit in US District Court in San Francisco alleges that AT&T hit unlimited data customers with an "unfair mobile data throttling program" and that AT&T committed a "deceptive failure to disclose [the] mobile data throttling program."

AT&T denied the allegations, saying that its practices are similar to those of other carriers and that it has been "completely transparent with customers since the very beginning."

The FTC's lawsuit described AT&T's actions as follows:

The speed reductions and service restrictions in effect under Defendant’s throttling program are not determined by real-time network congestion at a particular cell tower. Throttled customers are subject to this reduced speed even if they use their smartphone at a time when Defendant’s network has ample capacity to carry the customers’ data, or the use occurs in an area where the network is not congested. Once customers have been throttled during a given billing cycle, Defendant caps their download speed until the end of the billing cycle, at which time Defendant restores the data speed for these customers to full speed. Since October 2011, Defendant has throttled its customers more than 25 million times, affecting more than 3.5 million unique customers. When a customer is throttled, the customer’s data speed is reduced, on average, for the last twelve days of the customer’s thirty-day billing cycle. Defendant has numerous alternative ways to reduce data usage on its network that do not involve violating its promise to customers. One alternative would involve Defendant requiring existing unlimited data customers to switch to a tiered data plan at renewal. Defendant considered and rejected this approach in part because of concern that renewing customers would switch providers rather than switch to one of Defendant’s tiered data plans. Another alternative would involve Defendant introducing its throttling program at renewal, with disclosures at point of sale. Defendant considered and rejected such an alternative, in part because it “[a]pplied to all customers” and would not let Defendant “isolat[e] communications to [the] heaviest users.” Yet other alternatives might include limited, narrowly tailored throttling programs that are consistent with Defendant’s contracts, advertising, and other public disclosures. At the same time that Defendant has been throttling unlimited mobile data plan customers who exceed 3 or 5GB of data usage during a billing cycle, Defendant has been offering individual tiered mobile data plans for data usage of at least 30GB per billing cycle. Defendant does not throttle its tiered mobile data plan customers, regardless of the amount of data that a tiered mobile data plan customer uses.

AT&T's "throttling program has been severe, often resulting in speed reductions of 80 to 90 percent for affected users," the FTC said.

The FTC asked the court to issue a permanent injunction against AT&T to prevent future violations of the FTC Act. The commission also requested financial damages "to redress injury to consumers resulting from Defendant’s violations of the FTC Act, including but not limited to rescission or reformation of contracts, restitution, the refund of monies paid, and the disgorgement of ill gotten monies."

AT&T: “Allegations are baseless”

AT&T is fighting the allegations.

“The FTC’s allegations are baseless and have nothing to do with the substance of our network management program. It’s baffling as to why the FTC would choose to take this action against a company that, like all major wireless providers, manages its network resources to provide the best possible service to all customers, and does it in a way that is fully transparent and consistent with the law and our contracts," AT&T General Counsel Wayne Watts said in a statement sent to media. "We have been completely transparent with customers since the very beginning. We informed all unlimited data-plan customers via bill notices and a national press release that resulted in nearly 2,000 news stories, well before the program was implemented. In addition, this program has affected only about three percent of our customers, and before any customer is affected, they are also notified by text message.”

Federal Communications Commission Chairman Tom Wheeler has also been pressuring carriers over throttling of unlimited data. Verizon Wireless abandoned a plan to throttle certain LTE users after Wheeler's objections, though it still throttles heavy 3G users on unlimited data plans. Wheeler also sent letters to AT&T, T-Mobile, and Sprint expressing similar concerns about their throttling programs.

The FTC would not say whether it plans similar lawsuits against the other national carriers. "Since FTC investigations are non-public, we can’t confirm or deny whether we’re investigating any other company," a commission spokesperson told Ars.