If any of the victims' checks were deposited at your bank, that would have led investigators to approach your bank with a subpoena for your customer's account records. Of course, it is also possible that not all victims were identified.
Either way, it's right that the negative news discovery is prompting you to complete a due diligence review of your customer's account. Ultimately, it's the result of that review which will drive whether or not your file a SAR. The question that I would be considering, as a reviewer, is whether or not I suspect that any of the deposited checks represent proceeds from the thefts. If my answer is "yes", and the threshold is met, I'd file.