The Senate is considering new legislation that could force Verizon Wireless to cease tracking mobile customers’ online activity with “supercookies” that have come under harsh criticism from privacy advocates. In letters to regulatory agencies Friday, a handful of senators threatened to draft bills and requested investigations into Verizon’s policy.

“This whole supercookie business raises the specter of corporations being able to peek into the habits of Americans without their knowledge or consent,” U.S. Senator Bill Nelson (D-FL) said in a statement. “That’s why I think we need to get to the bottom of this and perhaps new legislation.”

The calls for an investigation, to the Federal Trade Commission and Federal Communications Commission, come after complaints from cybersecurity experts and consumer advocates that Verizon is monitoring its more than 100 million customers without their consent or knowledge. Also known as “perma-cookies,” the files act as unique identifiers that track every website users’ visit as well as their app activity, even after they try to delete records of their online activity.

Not only does the tracking method create serious privacy concerns, opponents have said, but Verizon has also failed to ensure that its customers are adequately protected. One prominent researcher at Stanford University told the New York Times that, as of Jan. 26, “Verizon is not in a position to control how others” use the information the wireless company provides, creating a security flaw that could be exploited by an intelligence agency or malevolent advertiser without much difficulty.

The company said last week it would allow customers to opt-out of the tracking program. Verizon should instead employ and opt-in program, Nelson said. “As we consider whether legislation may be necessary to fully protect consumers from the use of these supercookies, we also believe the Federal Communications Commission should use its full existing statutory authority to examine these practices,” stated Nelson’s letter to the FCC.

The letter was also signed by Senators Edward Markey (D-MA) and Richard Blumental (D-CT).