How do i find out which bank owns a house???

I have found three different houses that all have been vancant for over 2 years or more. So I did my home work and checked the public records at the county tax office an found out the previous owners names and wrote them letters, only to have them sent back return to sender. After some more research I found out that these properties are bank owned but I cannot figure out how to find out which banks own them. Does anyone know any further steps I can take to find out which banks own them??? Also the reason why I know they are bank owned is because there were numbers left on the front doors which I called an was told that they managed the properties for the bank, but they all told me they didnt know which bank owned the property. If anyone has any information that could help that would be great. Thanyou

Joe