Two U.S. senators sent a letter to the Federal Trade Commission and Justice Department about concerns that DraftKings and FanDuel are conspiring to maintain a duopoly over the sports gambling and daily fantasy markets.

Senators Mike Lee (R., Utah) and Peter Welch (D., Vt.) penned a letter Thursday to FTC chair Lina Khan and assistant attorney general of the DOJ’s antitrust division Jonathan Kanter, which Lee posted to social media Friday. The letter says it “seems that FanDuel and DraftKings have arguably acted as one company” since the FTC, state of California, and Washington, D.C., sued in 2017 to block their merger attempt, and the companies called it off. The legislators ask the regulators to “undertake any investigative or enforcement actions necessary” to make sure the companies aren’t violating antitrust law outlined in the Sherman Act.