Finding owner of distressed property

[Michael Palmore]

I am new to wholesaling and did everything step by step. I called up the cash money buyers and got s list of the type of properties they are looking for and I started looking for the distressed properties that fit their needs. I wrote up my own version of the "yellow letter" and used the cities assessors office website to find the owner of the house. I drop off the letter to the address they have on file and the guy called me back 10 minutes later saying he hasn't owned the house in 7 years. So how can I find the owner of the house when the city does not know? Btw I'm from Philadelphia