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Three Democratic lawmakers on the influential Senate Committee on Commerce, Science and Transportation are calling on federal regulators to investigate Verizon for its practice of using unique customer codes to track the online activities of its wireless subscribers.

The company uses the tracking technology — alphanumerical customer codes known as supercookies — to segment its subscribers into clusters and tailor advertising pitches to them.

Although Verizon allows subscribers some choices regarding the use of their information for marketing purposes, the company does not permit them to opt out of being tagged with the persistent tracking technology.

The senators sent letters on Friday morning to the Federal Trade Commission and the Federal Communications Commission, saying that the tracking technology may violate consumers’ privacy and asking regulators to investigate Verizon’s practices.

The letters come after a report by Jonathan Mayer, a lawyer and computer science graduate student at Stanford University, who discovered that Turn, a marketing software company, was exploiting Verizon’s supercookies to track consumers for its own marketing purposes.

Turn’s activities were first reported by ProPublica. Turn has since said it would suspend its use of Verizon’s supercookies, pending a re-evaluation of the practice.

After an article last week in The New York Times, the senators wrote a letter to Lowell C. McAdam, the chief executive of Verizon, asking the company to explain whether it had changed its policy after learning of Turn’s activities. The next day, Verizon announced that it intended to allow mobile subscribers to opt out of being tagged with the persistent customer codes.

But on Friday the senators criticized Verizon for failing to fully answer their questions. And one of the senators, Bill Nelson, Democrat of Florida, said he would rather the company obtained permission from subscribers ahead of time before saddling them with persistent identification tags.

Senator Nelson “said he would rather see consumers have to opt-in,” according to a news release from his office.

Debi Lewis, a Verizon spokeswoman, said: “Verizon takes our customers’ privacy seriously. We’re aware of the letters and will respond.”

Even before the senators’ letter, Verizon faced an uncertain future for its ad-targeting program.

Earlier this week, Tom Wheeler, chairman of the Federal Communications Commission, urged the commission to adopt the strongest rules possible for net neutrality, the principle that the Internet should be a level playing field for companies of all sizes.

Mr. Wheeler’s proposal recommends reclassifying Internet service providers as common carriers, akin to public utilities companies, which are subject to strict data-privacy rules.

If the rules are adopted, ad-targeting programs like Verizon’s could arguably become illegal, according to Harold Feld, a senior vice president at Public Knowledge, a nonprofit group that focuses on information policy.