Five Democratic US Senators are urging the Department of Transportation (DOT) to examine the “potentially anti-competitive” restrictions some carriers place on third-party price comparison websites.

In an Aug. 30 letter to DOT secretary Elaine Chao, the lawmakers—including Democratic presidential candidate Elizabeth Warren—requested the department reinstate an earlier proceeding exploring industry practices related to the comparison services that was suspended in March 2017.

In October 2016, DOT issued a request for information (RFI) seeking to understand how carriers may be restricting “distribution or display of airline flight information,” and whether that constitutes an unfair method of competition. More than 60,000 comments were submitted before the department suspended the proceeding, reasoning that more time was needed for the new administration to review the RFI.

“Over two years later, the RFI remains suspended, and DOT has not taken meaningful action to study, let alone correct the airline industry’s attempts to stifle marketplace competition and consumers’ ability to price shop,” the senators wrote. “The traveling public depends on third-party price comparison sites to make apples-to-apples comparison among fares and flights and to select the best price, schedule and airport from all available options.”

The lawmakers specifically mentioned Atlanta-based Delta Air Lines’ practice of prohibiting third-party price comparison websites, including online travel agents (OTA) and metasearch websites, from fully accessing the carrier’s data. They also cited Dallas-based Southwest Airlines for completely withholding its information from travel websites.

“A consumer wishing to see how Delta’s prices and schedules stack up against other carriers on the OTA Trip Advisor, or a metasearch site like Hipmunk, would be out of luck,” the lawmakers wrote.

Delta spokeswoman Lisa Hanna told ATW  that Delta works with many travel agents and metasearch sites that support online distribution, adding that the carrier avoids doing business with services that don’t meet its quality standards.

“Delta chooses not to do business with certain online travel sites that create poor itineraries, fail to disclose important information, make false or misleading representations or engage in outright fraud.  In such instances, we have an obligation to protect our customers and the Delta brand.”

A spokesperson from Southwest did not respond to a request for comment.

Ben Goldstein,