I can understand, you where trying to prove a negative, which in this case is next to impossible and thus cannot really give a good argument even if it is true.
Anyway I remembers this, https://www.fairus.org/issue/societal-impact/noncitizens-voting-violations-and-us-elections and https://www.sciencedirect.com/science/article/abs/pii/S0261379414000973
Not the same type of fraud, but it is enough to swing close elections.