Verizon Wireless's use of so-called supercookies is coming under scrutiny, and Democratic senators on Friday called on federal regulators to investigate the unique tracking codes, the Associated Press reported.

The nation's biggest mobile provider attaches the identifying strings of letters and numbers to each site visited on a customer's mobile device, and privacy advocates have long criticized the practice, according to CNET. They say that supercookies are hard to circumnavigate and violate personal privacy as they collect and analyze data about online activities.

Regular browser "cookies" are tiny bits of text stored by users' browsers that contain information about the websites they visit and how long they remain on a given page. Unlike the traditional version, the kind used by Verizon can be more difficult to detect and cannot be easily deleted.

In a letter sent to the Federal Communications Commission (FCC) and the Federal Trade Commission (FTC), Sens. Bill Nelson of Florida, Ed Markey of Massachusetts and Richard Blumenthal of Connecticut are now asking for an investigation of the trackers.

The FCC, an independent government agency, is charged with regulating interstate communications by radio, television, wire, satellite and cable across the entire U.S. territory. The FTC, meanwhile, aims to promote consumer protection and eliminate anticompetitive business practices. Both agencies are headquartered in Washington.

"This whole supercookie business raises the specter of corporations being able to peek into the habits of Americans without their knowledge or consent," Nelson, who serves as the top Democrat on the Senate Commerce Committee, said in a statement. "That's why I think we need to get to the bottom of this and perhaps (need) new legislation."

A spokeswoman for Verizon Wireless, which has more than 100 million customers in the United States, said the company planned to write to the FCC and FTC, the AP noted. Verizon had announced last week that it would give users the option to opt out of the supercookie program.

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

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