Who betrayed Anne Frank and her family?

The franks were caught because the thief that came in the office room was eventually caught by the police and in order to save himself from going to jail he bargains with the Nazi telling them that he believes that there are Jews hiding in a secret room behind the book shelf of the office. That is how the franks were eventually caught. Mr.kraler couldn't warn them because he was captured by the police and was at gun point and they forced him to show them were the secret annex was.
No one knows who told on her family and we'll probably never know.