In a letter to FCC Chairman Ajit Pai, three US senators are calling for an investigation into whether US mobile carriers are throttling traffic to certain streaming apps like YouTube and Netflix without telling customers.

The letter, sent Wednesday by Democratic Sens. Edward Markey of Massachusetts, Richard Blumenthal of Connecticut and Ron Wyden of Oregon, said that throttling could be a violation of Federal Communications Commission transparency rules. They cite a study from Northeastern University and the University of Massachusetts at Amherst that almost every US cellular company throttles at least one streaming video service.

Markey, Blumenthal and Wyden said that upon reaching out to the carriers in question, they didn't receive sufficient explanation for what was going on.

"The lack of clear and complete information that the carriers provided in response to congressional inquiries should prompt the Commission to investigate the carriers' practices and determine if they violate the existing transparency rules," the letter said. The senators are asking for a response from the FCC by Feb. 27.

This comes after the 2017 vote to repeal net neutrality, the Obama-era rules that kept broadband providers from slowing or blocking access to the internet or from charging companies to get faster access. 

The FCC, YouTube and Netflix didn't immediately respond to a request for comment.