Unless you had auditing turned on you most likely are out of luck. Hopefully you can recover these files from a backup or archive. If this continues to be an issue you may need to address the issue of your system authority. Turn on auditing for starters. If these are key file to your business you may want to limit who has access to commands like DLTF. Check other possible area such as SQL using DB2 databases. Are any applications using DROP TABLE. If this is a fairly new issues, I have seen it though, is may have been done by a disgruntled employee. If anyone has been let go recently make sure they do not have a valid sign on..