Two U.S. senators want to know what Steven Mnuchin and his Treasury Department are doing to counter the “nefarious” use of cryptocurrency such as Bitcoin by Venezuela, Russia, and North Korea.

Sens. Marco Rubio, R-Fla., and Bob Menendez, D-N.J., sent a letter to Mnuchin on Friday warning that Venezuela President Nicolas Maduro was planning to create his own digital currency, called the “petro” after the country’s oil wealth. They asked whether Treasury can modify existing sanctions on Venezuela to block Maduro.