Presently, Citibank stock is trading at a level that implied an almost imminent bankruptcy. Most likely, on the basis of assets and liabilities, properly valued, it is insolvent.
What also complicates matters is that it is a global institution with deposits in non-dollar currencies. That would complicate the task of making depositors whole in case of its collapse.
What surprises me is that, as far as we know, it is not yet experiencing a bank run. If I was a client of Citibank, especially a foreign client but also a domestic, I would likely withdraw the money even if I was covered by FDIC guarantee, but certainly would do so if I was not covered.