I am not a lawyer.
If there are no passwords on the systems, I seriously doubt you have any chance of proving who was responsible. There is a world of difference between suspecting and proving in a court of law.
If the material was of an illegal nature you should call in the police. Failure to do so makes you and your company complicit. The more the information is examined, the more the evidence is corrupted. If the material is illegal, call the police immediately. I'm sure if your perpetrator is still working at the company, having the police take a computer away for forensic examnination will, at least, stop them from viewing such material.
This would also be an ideal opportunity to suggest to your company that they need to take the security of their systems seriously. I'm sure they have customer data on these computers and I doubt they would continue to be happy customers if they were aware of how their information was being handled.
I do not know which country you are from and the laws concerning indecent material vary from county to country as do the laws concerning computer misuse and investigation.
I am not a lawyer but if you are in England or Wales, I can provide you with the advice you need from a legal perspective. If not, consult a lawyer and probably even if you are in England or Wales!
Did I mention I am not a lawyer?
CISSP, C|EH, C|HFI