I would agree with gentle lemurs comments , personally I doubt it was keeper error , even briefly not closing the door it would take some time for all 20 to realise and escape! One or two you could understand perhaps(?). My guess is they dug under the fenceline somehow , though to be fair they are not really renowned for digging on a large scale like say Timber Wolves for instance. Its a real shame , Howletts have had alot of success over the years with this species, but equally they have had outbreaks of disease within the group which have been devastating to numbers in the past. Group numbers have fluctuated over the years as a result. They are a highly strung species , somewhat nervous but also quite aggressive for their physical size. I hope the missing animals are found quickly. Z.