Senate Democrats are asking federal regulators to investigate Verizon's use of "supercookies" that track online activities.

Spencer Platt/Getty Images

On Friday, Sens. Bill Nelson (D-Fla.), Ed Markey (D-Mass.) and Richard Blumenthal (D-Conn.) sent letters to the Federal Communications Commission and the Federal Trade Commission asking the agencies to look into Verizon's use of tracking technology.

The senators along, with Sen. Brian Schatz (D-Hawaii), had written to Verizon CEO Lowell McAdam last week to express their concern regarding reports that a third-party advertising company called Turn was using Verizon's "supercookies" to gather information for targeted advertising.

Browser "cookies" are tiny bits of text stored on your device by your browser. They contain information about the Web sites you've visited and how long you stayed on those sites, along with other information about your browsing activities. "Supercookies" are cookies designed to be permanently stored on devices. They're often more difficult to detect than regular cookies and they can't be deleted in the same way regular cookies can.

Privacy advocates have criticized the wireless operators for using this technology, charging that its use is a violation of personal privacy since supercookies collect and analyze data about online activities and are hard to do away with. Privacy advocates also warn this information could be used by hackers to track users' activities.

Turn has suspended the use of supercookies on Verizon subscribers' devices. And Verizon responded to concerns about the use of the technology, stating that it is working on a fix that will allow customers to opt out of the use of supercookies.

But the lawmakers say they're still concerned with how the technology is being used. And they're considering legislation to limit its use. In the meantime, they're asking the federal agencies to investigate the use of supercookies to see if such use violates regulations or laws.

"This whole supercookie business raises the specter of corporations being able to peek into the habits of Americans without their knowledge or consent," Nelson said in a statement. "That's why I think we need to get to the bottom of this and perhaps [adopt] new legislation."

Verizon spokeswoman Debra Lewis acknowledged the company is aware of the senators' concerns.

"Verizon takes our customer's privacy seriously," she said in a statement. "We're aware of the letters and will respond."

The lawmakers' request for investigations follows reports last year that AT&T and Verizon were using supercookies to track the activities of their smartphone customers. AT&T stopped using the technology in November, according to the Associated Press. The lawmakers' request also comes as several reports indicate Verizon's supercookies have been used to track the Internet activity of millions of its customers, even if those customers had taken steps to delete their cookies.

Last week Verizon said it would start allowing customers to opt out of the use of supercookies. The company has put together a FAQ to help answer consumers' questions.

In the company's response last week to lawmakers, Verizon Executive Vice President Craig Silliman explained how the company is protecting consumer privacy.

"We never share information that individually identifies our customers with third parties, and we give customers appropriate choices about whether and in what circumstances they will see advertising that is tailored to them," Silliman said. "We are also sensitive to concerns raised by our customers, and we make changes to our programs to address their concerns. For example, last week we announced that we are implementing a process to automatically disable the UIDH ["Unique Identifier Header" -- aka the supercookie technology] for customers who opt out of our advertising program."

In spite of this action, lawmakers don't seem satisfied with Verizon's response. And in their letter to the FTC, they specifically ask the agency to review Verizon's privacy disclosure policy and to investigate whether Verizon has taken steps to shore up consumer privacy after the company discovered how its partner Turn was using the supercookie technology.