"FBI agents visited Berg's parents in West Chester on March 31 and told the family they were trying to confirm their son's identity."
of all sources:
http://www.foxnews.com/story/0,2933,119734,00.html
but they of course would have had to find him out on the ground, so chances are he was detained, why else would the FBI be "trying to confirm his identity?"
they released him only after his parents filed a lawsuit.