I looked at one of the guilty pleas linked in the article (https://media.scmagazine.com/documents/290/vartanyan_guilty_...), hoping that it might contain more details concerning how he was caught.

Unfortunately, that information is not included. However, the plea has a section about "removal from the united states" (on page 4). I found this highly amusing, considering they went to great lengths to get him to the US in the first place.