I can't see why you would be responsible for returning any of the security deposit if they did more damage then the amount of the security deposit -even if the notification wasn't received by the previous tenants within 21 days.
As others have stated, I'm not an attorney but if it were me I would send them a notification in writing now detailing exactly what the damages are and how much it will cost to fix them. I would include copies of any invoices and estimates of repairs demanding immediate payment or that I'll need to take them to small claims court.
Past experience tells me that they most likely won't pay me so I would need to file a claim with conciliation court and let the judge sort everything out.
Check out the Landlord Tenant Rights and Responsibilities information on the Minnesota Attorney General's website.
If it turns out that the management company was in fact in the wrong, do everyone out there a favor and file a complaint with the commerce department against the licensed agent running the company. The first complaint won't result in much punishment but if they do it again they will start to get in trouble and eventually lose their license. It may sound mean but as licensees we need to be held to a higher standard in order to regain the trust of the public.
Coldwell Banker Burnet
Licensed MN Broker