Senior Democratic leaders of the U.S. Senate are investigating the relationship between Tether and investment bank Cantor Fitzgerald.

Richard Blumenthal, the top Democrat on the U.S. Senate Permanent Subcommittee on Investigations, is demanding Wall Street investment bank Cantor Fitzgerald explain its relationship with Tether, the world’s largest stablecoin issuer. If Democrats make further gains in the November elections, Blumenthal and his colleagues could soon gain expanded investigative authority. On Thursday, Blumenthal sent a letter to Cantor CEO Brandon Lutnick, son of Howard Lutnick, who served as U.S. President Donald Trump’s Commerce Secretary and handed over management of the firm to his son after joining the administration. Blumenthal has been investigating Tether’s role in Iran’s financial system, and noted that Tether, the issuer of USDT, has a lucrative business relationship with Cantor Fitzgerald, which manages its U.S. reserve assets. In the letter, Blumenthal wrote: “Just as Tether has reaped incalculable interest income and investment gains from stablecoins used in these illegal activities, Cantor Fitzgerald has also profited from its partnership with Tether.” He also asked Brandon Lutnick to explain the firm’s banking operations and sanctions compliance safeguards, and inquired about his father’s prior dealings with Tether during his tenure at Cantor.