I worked in the field of investigations for four years. Being "investigated" for filing fraudulent expense reports and being "guilty" of filing expense reports are two completely different entities. If you are guilty of doing this (as they may determine through an investigation), it really will come down to the severity of your offense. I don't know what line of work you are in, but if it's minor, you may only face a write-up or termination. In severe cases, you could face jail time.
Is your company investigating you internally or was an investigation company called in to assist? If it's the latter (which is what I did), you should definitely at least talk to a lawyer to get a legal opinion and explain your situation in depth. Often times companies will use scare tactics to get a confession when they have no supporting evidence or maybe the evidence they do have could easily be disputed. If you are worried about your job (and it sounds like you are), I would absolutely seek some form of legal assistance. It doesn't hurt to at least call and ask, and it could mean all the difference. Hope this helps. Good luck!