I don't believe it has been confirmed. I can only find reports of his "multiple identities + passports."IIRC, it was mentioned before that the husband has a dual citizenship (also nationality and citizenship aren't the same). He likely wasn't a US citizen at the time he got married at 22. He went to university in Europe.
Also, this article uses the phrase "David, of Serbian origin"
Three weeks since the disappearance of Ana María Knezevich in Madrid: what authorities know so far
Ana María Knezevich has been missing for two weeks, with no trace on her bank or phone records. What we know so far...
en.as.com