You need to consult legal counsel, but I don't think you can just hold on to the money. When you say altered checks, what was altered and should you have know at the time of deposit and you are sure the bank has no liability? If so:
The depositor should be given access to the money and also let them know that they probably need to engage their own legal counsel. As if they spend the money, then they may become liable for unjust enrichment, if they fully know the funds were obtained through a fraudulent means.
They need to work that out with their own legal counsel. The bank is neither judge or jury in this case.
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