ABC described the federal probe as a "wide-ranging" and related to a federal grand jury in Washington. While several U.S. businessmen are reportedly involved, it's unclear whether Cuellar is target for the grand jury, the source reportedly told ABC.
The office of Cuellar, a moderate Democrat whose district includes a swath of San Antonio, has said he's cooperating with authorities.
A separate investigation by online news site The Intercept catalogued Cuellar's links to Azerbaijan, reporting that he "has had close ties" with the Assembly of the Friends of Azerbaijan, a group previously investigated by the FBI. That organization's president pleaded guilty in 2018 to charges of soliciting help from Congress while representing Azerbaijan's wholly owned oil and gas company, according to the story.
After the trip, Cuellar spoke at a 2013 Washington reception in honor of the State Oil Company of Azerbaijan Republic (SACAR), noting the importance of a pipeline to deliver natural gas to Europe, according to The Intercept. That same year, he and others in Congress sponsored a resolution supporting just such a pipeline project.
What's more, The Intercept reports, Cuellar in 2015 announced an affiliation agreement between Laredo's Texas A&M International University and the Assembly of the Friends of Azerbaijan, which the congressman said in a press release was an “educational and cultural organization.”
Last year, a section of SACAR's commercial pipeline began delivering gas from Azerbaijan to Italy, according to the story.
Cuellar reportedly didn't respond to a request for comment on The Intercept report.
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