The check casher could have endorsed the back of the checks, and then anybody could have cashed the checks, or better yet, he deposited the checks into the account, and the company that wrote you the checks would have been none the wiser. the crook then withdrew the money from his account, and skipped town.
The crook must have gotten the mail from the company that was sending the checks, or he was stealing mail from your company. There may have been a insider in your company or your customer's company that were stealing the checks. Were all the checks written by 1 customer, or different customers. This may help to answer the question as to how the crook got the checks. if it was from different companies, then the theft took place on your end.
I am assuming you contacted your customer, and asked them about the unpaid bills. they probably told you that they sent the checks, and you cashed them.
Each customer MUST file a fraud affidavivdt with their bank, and ask for the money back. There could be a problem with this however, all fradulatent activity must be reported within 60 days or the bank has no liability. On your post you give no indication about when the fraud took place.
Once your customer(a) get their money back, they can reissue you new checks, end of story. If your customer(s) waited too long to file a fraud affidavit, and their bank(s) refuse to refund the money, you will have no choice but to sue in small claims court.
They will use as their defense, that they sent you the checks, and they were cashed, and now the problem is yours. But any Judge will deny this defense, and issue a ruling in your favor.
If these customers are good customers, and you sue them, you may not keep them as customers.
also remember that if you get a judgment against your customers, they may not pay anyway. Usually a judgment and a 1.00 will get you a cup of coffee.
In conclusion this is not BOA's fault. Someone had an account with BOA, and they brought checks for deposit, and the checks went through. many times I have been paid with checks that someone else had been written to someone else. They gave me the checks, and endorsed them over to me.
Sorry this is not the answer you wanted to hear