Bo Hines took the top job at Tether's U.S. arm after leaving his post as White House crypto adviser. Now U.S. Senator Richard Blumenthal has sent letters to Tether and Nxum Group, the consulting firm Hines co-founded with his father and another partner.

The letters follow reports that Nxum received large consulting payments from Tether while Hines was still advising the Trump administration on crypto policy, including rules on stablecoin issuers. Blumenthal, the ranking Democrat on the Senate Permanent Subcommittee on Investigations, requested details of those arrangements.

Blumenthal wrote that the deal terms "raise alarming ethical and legal concerns" and suggest Tether knew paying a sitting White House official was improper.

It is the latest in a series of Tether probes by the senator, after a report on Tether's links to Iran and questions over its ties to Cantor Fitzgerald. Tether CEO Paolo Ardoino has criticized the investigations, and the company did not immediately comment.