A trio of Democratic senators are pressing mobile carriers to answer questions about their throttling practices following a study that shows them slowing down their networks.

The study they cited, conducted by the app WeHe, found that Verizon, AT&T and Sprint throttled web traffic from a range of popular video services such as YouTube, Netflix, Amazon Prime, NBC Sports and Skype.

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in their letter

to the three telecommunications companies.

The senators asked the companies to provide more details on their internal practices and service management to address potential throttling.

Markey on Thursday tweeted that the results of the Wehe study highlight the need for net neutrality rules. The Federal Communications Commission under its Republican chairman moved to scrap the rules late last year.

Democrats in Congress are still pushing for legislation that would reinstate net neutrality but are not close to a deal with Republicans on the terms of such legislation.

A representative for CTIA, a D.C. based trade association that lobbies on behalf of wireless companies, including Verizon, AT&T and Sprint, said that the throttling was the result of “wireless network management.”

“Wireless providers allow consumers to choose from a range of options, and providers manage their networks to meet consumer needs,” said Tom Sawanobori, CTIA’s senior vice president and chief technology officer.

“By failing to take into account consumer preferences and basic wireless network management, the Wehe study offers little insight into the U.S. consumer wireless experience.”