Hello from Seattle. Need help with carpets.

I work in seattle cleaning service. Our clients just bought an older house and the people who lived there before me had several dogs and cats in the house.  apparently, they people didnt know that dogs potty outside and cats use a litterbox or go outside.  the carpets are completely disgusting.  what can I do to get that smell out?  i have had them shampooed and steam cleaned, but that smell is still there.  i really dont want to have to rip up the carpets?  can they be saved?