Multiple (Nested) Codeigniter apps not working in NGINX

We've setup 2 CodeIgniter apps which has its own complete code bases on its own directories as in this example. Plus we also have wordpress blog on the same public_html directory.

public_html/HOME_APP
public_html/admin_tool/ADMIN_TOOL_CODES
public_html/blog/WORDPRESS_CODES
HOME_APP codes (CodeIgniter) and our WORDPRESS works fine. But the admin_tool (CodeIgniter) doesn't work. We can access only the http://example.com/admin_tool/index.php but not any inside controller pages. When accessing its show 404 error page. And it looks like the routes are handled via APP1

The nginx rules are as follows. Appreciate if anyone can help us to fix the issue with admin_tool

server{
listen 80;
root /home/ubuntu/websites/example.com/public_html;
index index.html index.htm index.php;
server_name example.com;
access_log /home/ubuntu/websites/example.com/logs/access.log;
error_log /home/ubuntu/websites/example.com/logs/error.log error;

location ~* \.(ico|css|js|gif|jpe?g|png)(\?[0-9]+)?$ {
expires max;
log_not_found off;
}

location / {
index index.php;
# Check if a file or directory index file exists, else route it to index.php.
try_files $uri $uri/ /index.php;
}



location ~ \.php$ {
try_files $uri =404;
fastcgi_pass unix:/var/run/php5-fpm.sock;
fastcgi_index index.php;
fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
include fastcgi_params;

}

location ^~/admin_tool/ {
root /home/ubuntu/websites/example.com/public_html/admin_tool;
index index.php;
try_files $uri $uri/ /index.php$args;
}