Two wolves have been shot dead and a third is on the run after escaping from their enclosure at Colchester Zoo.
Staff at the zoo found the perimeter fence damaged and five of the six timber wolves escaped.
One returned of its own accord, a second wolf was darted and caught and a further two shot.
The remaining wolf is thought to be sheltering in thick undergrowth, a spokeswoman for the zoo said. Essex Police is helping in the search.
"Colchester Zoo's keepers have been devastated by the loss of two of their beloved timber wolves," the spokeswoman said.
"Wolves are naturally timid and provided they are not cornered in any way do not provide a risk to the public."
Only the lower half of the zoo was closed off in response to the escape as a "precaution".
A police spokeswoman said ground officers and the force helicopter were involved in the search.