Overseeing the effort is San Antonio-based Assistant U.S. Attorney Joe Blackwell, according to a person familiar with the probe who insisted on anonymity because the investigation is ongoing.
Paxton has denied any wrongdoing. His unsuccessful recent lawsuit trying to overturn the presidential election has raised questions about whether he's seeking protection from a powerful ally -- President Donald Trump.
The U.S. attorney's office in West Texas declined to comment or make Blackwell available for an interview. Federal prosecutors and the FBI generally do not confirm ongoing investigations.
Blackwell, who coordinates public integrity cases across a region stretching more than 500 miles (800 kilometers) to El Paso, is a hard-charging but fair prosecutor, according to former colleagues and opposing lawyers alike. He has a knack for boiling down the complex facts of white-collar investigations, a down-to-earth demeanor and slight Louisiana accent that endears him to juries.