The letter follows recent reports about the practice, which many mobile-phone users are unaware of. In December, The New York Times reported that at least 75 companies receive anonymous but precise location data tracking up to 200 million mobile devices in the U.S., updating their whereabouts as many as 14,000 times a day. The data is sold or used by advertisers and hedge funds.

AT&T, Verizon, T-Mobile, and Sprint have vowed to protect the location data of their customers. But this month, Vice’s Motherboard said that location data is not only still available, but accessible to ordinary citizens, including stalkers, domestic abusers, and criminals.

“It is clear that these wireless carriers have failed to regulate themselves or police the practices of their business partners, and have needlessly exposed American consumers to serious harm,” said the letter, which was signed by senators such as Ron Wyden (D-Ore.), Chuck Schumer (D-NY), Kamala Harris (D-Calif.), Kirsten Gillibrand (D-NY) and Bernie Sanders (I-Vt.).

“To that end, we urge the FTC and the FCC to conduct broad investigations, as appropriate, into the business partnerships between wireless carriers and location aggregators,” the letter said. It also asked them to require the carriers “to notify every American whose location they shared or sold and to identify to those subscribers the specific companies that obtained their location information.”

The senators asked the FCC and the FTC to respond by Feb. 5. Both agencies are operating with reduced staff because of the partial government shutdown. FCC chairman Ajit Pai recently declined to attend an emergency briefing with a House committee to discuss the data-tracking issue, citing the shutdown.