Mark is a CPA in a large city who audited the financial statements of a firm that sold specialized computer equipment around the world. This was a complex audit, and Mark prepared numerous working papers in connection with the audit. Mark also had numerous conversations with client personnel in his efforts to understand the complex transactions and accounts. Mark's client has recently become the subject of an investigation for numerous violations of law. Charges will soon be filed at both the state and federal level. Can Mark be called to testify, and can Mark's work papers be subject to a subpoena?