A similar situation was documented here: https://m.youtube.com/watch?v=ponylQTj_gg&pp=0gcJCdgAo7VqN5t...
On the one hand the guy in OP article didn’t have documentation and he illegally crossed the border, so what do you do as ICE? The guy claims he is a US citizen, but I bet you a lot of illegal immigrants without documentation claim they are a US citizen as well.
Also there is no federal ID system, so how do you go about confirming if this person is a US citizen or not? It does seem reasonable that people within ICE custody should get the chance to call someone so that person can bring identification for ICE to confirm the identity, and that is maybe the missing part which lead to this situation.