FILE PHOTO: Ajit Pai, Chairman of the Federal Communications Commission, speaks at the WSJTECH Live conference in Laguna Beach, California, U.S. October 21, 2019. REUTERS/Mike Blake

WASHINGTON (Reuters) - U.S. Federal Communications Commission (FCC) Chairman Ajit Pai said on Friday the telecommunications regulator plans to take action against at least one unnamed wireless carrier over the apparent unauthorized sale of real-time location data from users.

The FCC said in May 2018 it was referring reports that a website flaw could have allowed the location of mobile phone customers to be tracked to its enforcement bureau to investigate. In a letter to Congress on Friday, Pai said the FCC’s enforcement bureau “has concluded that one or more wireless carriers apparently violated federal law.”

FCC Commissioner Jessica Rosenworcel said on Friday it was a “shame” the FCC took so long to act on what she called reports that “shady middlemen could sell your location within a few hundred meters based on your wireless phone data.” She added, “It’s chilling to consider what a black market could do with this data.”

AT&T Inc T.N and Sprint Corp S.N declined to comment on the FCC letter. Verizon Communications Inc VZ.N and T-Mobile US TMUS.O did not immediately respond to requests for comment.

A trade group representing U.S. wireless carriers said on Friday that “upon hearing allegations of misuse of the data, carriers quickly investigated, suspended access to the data and subsequently terminated those programs.”

Lawmakers last year expressed outrage that aggregators were able to buy user data from wireless carriers and “selling location-based services to a wide variety of companies” and that data could be obtained by bounty hunters and others.

Pai said he soon plans to circulate to the five-member FCC commission a formal notice of liability to one or more carriers.

A security researcher said in 2018 that data from a California-based tech firm could have been used to track mobile consumers of AT&T, Verizon, Sprint and T-Mobile US within a few hundred yards of their location and without their consent.

U.S. Senator Ron Wyden in 2018 told the FCC that wireless carriers were selling customers’ location data “to a shady prison phone company” that was allowing prison guards to track Americans’ cell phones. He said on Friday he was “eager to see whether the FCC will truly hold wireless companies accountable, or let them off with a slap on the wrist.”