finding the html control from code behind

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ASP.NET

HTML

in page load i add some html controls(put runat="server" attribute too) in a string builder and assign this string builder to a div from code behind..

now next time in a asp.net button click method i want to add more html controls in the above created one 'div' html control.so i try to find the div by id(which i assigned in page load as i said) by FindControl method so that i can add more controls in its innerhtml but cant find....plz help. how can i find dynamically created html div control(in page load) in button click method ?