Possible fraud relating to a discrepancy in accounts payable

You have been assigned to investigate possible fraud relating to a discrepancy in accounts payable in which management has informed you they suspect the new AP supervisor is involved because the balance in accounts payable has increased by 30% over the past year and the assistant to the AP supervisor has been acting "suspiciously". What type of evidence would you gather both from within the company and outside of the company to support management's theory? How would you determine if the evidence you gathered is relevant and reliable? How would you obtain the evidence based on applicable laws and professional standards? Be sure to back up your opinions with authoritative sources (including peer reviewed articles from the library, Fraud Examiners Manual, etc).