I think I may be held responsible for embezzlement, what can I do?

I have a situation where small amounts of money have been taken from work deposits & then covered up in the computer. I work at a doctor’s Office, and we are currently gathering evidence. It looks like this has been going on for years, but hidden. How much proof do I need, in order to prove beyond a reasonable doubt who did this? There are no cameras, no eye witnesses; different people were logged into the system when the changes were made, etc. Of course, all the staff members say they don't know anything. As the manager, I am afraid now after seeing how difficult this investigation is becoming, that I will be blamed for this. I fear that the investigation will come to a dead-end & when there is nobody else to pin it on, it will all fall on me. I am afraid that consulting an attorney will make me look guilty. What should I do?