This can be one of the most costly situations an employer encounters. Every company should have policies in place that clearly spell out what would be a breach of policy as well as what the penalties would be in the event it occurs. If you incur an internal theft it is important to do several things such as limit the number of people who will know about it and immediately hire someone to investigate it thoroughly. Both the company and the investigators should be careful to proceed slowly, as a rush to judgment to accuse someone can have devastating legal effects if it turns out to be wrong. A defamation lawsuit is hard to win if someone is wrongly accused.
You should, of course, immediately consult with an employment attorney. Part of that discussion should be to consider all the facts as you know them and then develop a strategy for the investigation. Over the years, Dave Culbreth Investigations has done numerous successful fraud and theft investigations involving corporations and businesses.