Citibank does not sell its own properties, they hire assett management companies to sell their properties. By looking on their website, they do list on their who has been assigned the property.
If it is not on there, check the house for when they post a notice with a contact for is selling the property, then contact them. While you are waiting make sure you have been prequailified as you will need a prequailification letter to even make an offer.
for mor etips on buying bank owned properties please check my blog.