Five of the six timber wolves at Colchester Zoo escaped on Tuesday at about 07:30 GMT. Three were shot dead.
Colchester Council and the zoo have carried out an investigation into what happened and ruled out foul play.
A zoo spokeswoman said managers were "struggling to comprehend" how the wolves had broken through the fence.
She said senior management had "complete faith" in their "competent and experienced staff to continue carrying out these duties as a daily procedure".
Essex Police also investigated how the timber wolves escaped and found no evidence of "deliberate damage".
The force learned of the escaped wolves at 08:00 GMT but did not confirm the escape to the media until about 14:00.
The zoo defended its decision to remain open until about 15:00 saying the lower end of the zoo - where the wolf enclosure sits - was sealed off to the public creating a "solid void".
The spokeswoman said the area was "continuously monitored by spotters and managers stationed around the perimeter" to protect staff and public at this time.
Sue Thornton of the International Zoo Veterinary Group, consultant vets to Colchester Zoo, said the decision to use "lethal force" on the wolves was correct.
"The delay of the effects of the tranquiliser coupled with the danger to the public of a category one dangerous wild animal - which a wolf is - means that sadly lethal force is appropriate once the animal has breached the zoo perimeter."
A pack of wolves which escaped from a zoo enclosure broke through a steel wire fence despite it being checked daily, keepers have said.