How did they look? As in were they just all laying around dead but untouched? Any marks at all on them? If they were in good health and then suddenly all died off at the same time, with no marks of trauma or anything like that I'm thinking it must be some sort of toxin they all either ingested or inhaled at the same time. Was the litter old enough to eat food or still exclusively nursing? Any chance anything was sprayed recently near them or may have gotten on their food? Any change in food recently?