Criticism from the chairman of the Federal Communications Commission over Verizon's new plan to slow download speeds in some cases is adding to concerns about data-use plans being a possible threat to customer rights and net neutrality.

Wireless and Internet service providers are already on the defensive as FCC Chairman Tom Wheeler considers new regulations to uphold the net neutrality concept that all digital traffic should be treated equally.

In a Wednesday letter to Verizon, Wheeler said he was “deeply troubled” by the company's July 25 announcement that it would slow or “manage” the speeds of some of its users.

Verizon’s policy for its advanced 4G network means the company may slow the data speeds of the top 5 percent of data users who are on unlimited plans and are using high-bandwidth applications when a cell site is under high demand.

“If you're on an unlimited data plan and are concerned that you are in the top 5 percent of data users, you can switch to a usage-based data plan as customers on usage-based plans are not impacted,” Verizon has said, according to Wheeler's letter.

Wheeler asked Verizon in the letter to explain how this does not violate rules requiring common carriers to “not deny, limit, or restrict the ability of end users to download and utilize applications of their choosing,” and whether this would be justified under net neutrality rules that also prohibit such interference.

“’Reasonable network management’ concerns the technical management of your network; it is not a loophole designed to enhance your revenue streams,” Wheeler said in the letter.

Verizon said Thursday it plans to respond to Wheeler's concerns, and called its announcement "a highly targeted and very limited network optimization effort."

"The purpose is to ensure there is capacity for everyone in those limited circumstances, and that high users don't limit capacity for others," the company said in a statement.

While the chairman’s net neutrality proposals have been criticized by Democrats for not going far enough to protect rights on the open Internet, Wheeler has said he will guarantee "people will get what they pay for," and his letter reflects a commitment to customer rights.

But ensuring people are getting what they pay for is a tricky concept if customers are overpaying, which is what a full report from the Government Accountability Office may show in November. Preliminary results of the study were revealed on Tuesday by Rep. Anna Eshoo, D-Calif., who is the ranking Democrat on the House Communications and Technology Subcommittee and a top proponent of strengthening Wheeler’s proposed net neutrality rules.

The study featured interviews with telecom analysts and assessments of the top wireline and wireless Internet service providers. It showed that customers may not be using as much data in their daily lives as they expect, which may lead them to overpay for services.

The study said the top four wireless companies used data caps to limit downloads, that three of those companies charged fees for people who exceeded those limits and that one provider slowed access in such cases. The study also showed that seven of the 13 Internet providers studied also used data caps to limit downloads.

Eshoo, who commissioned the study, called usage-based pricing a “new threat to the free and open Internet” in a letter to Wheeler this week.

“These new business models have left consumers wondering whether they will have to foot the bill and how much more it will be,” Eshoo wrote in the letter.

The FCC is reviewing public comments on the proposed net neutrality rules, which some Democrats like Eshoo have criticized for containing a possible loophole that could allow Internet providers to charge for faster access.

Despite this new wave of scrutiny, it is unclear whether Wheeler – a former telecom and cable industry advocate – would toughen his approach to net neutrality, says John Bergmayer, senior staff attorney at the consumer advocacy group Public Knowledge.