father blamed for illegal transactions on LIC what can he do now.

My father is currently working in LIC of India... And what actually happened with him is that they have their own account in LIC so somebody misused his account as he knew his password nd transacted multiple amount by the name of my mother... We really don't know about this thing that who actually did it...One letter came from my father's office that he has transacted the money intentionally and so on...So they are asking for valid reason that why and what made him to do these things....if the reply will not be valid they may take some action..So kindly help me out of it...What reasons should we give to them???