A call has been made for an independent inquiry into an escape by two chimps, one of which was shot dead, at Whipsnade Zoo.
Meanwhile, the zoo team made it clear that chimp Jonnie was killed "as a last resort" in the interests of public safety.
The 27-year-old chimp could not be recaptured and was shot dead after he and female chimp Coco, 44, went missing from an indoor den.
He was killed within the grounds, but Coco was safely recaptured just outside the zoo perimeter after scaling a fence on Bison Hill.
A call for an independent inquiry into the escape has now been made by the Captive Animals' Protection Society.
No staff members or visitors were injured during the drama, which happened while the zoo was open, just after 10am on Saturday, September 29.
The zoo has made it clear that it is carrying out its own full investigation into the incident.
Zoological director David Field said: "Initial attempts to recapture Jonnie were unsuccessful.
"He then moved out of reach from keepers and was therefore posing an immediate threat to the safety of the public and other staff."
And Mr Field went on: "The decision to shoot an animal is not taken lightly by the Zoological Society of London.
"In this situation, keepers were dealing with an unpredictable, strong and potentially dangerous animal.
"All options for the chimp's recapture were investigated. However, it was in the interests of public safety, and as a last resort, that he was shot in accordance with the zoo's procedure and policy for this kind of situation.
"Under the circumstances, this was judged to be the most appropriate and safest course of action."
Zoo visitors were told to remain in their cars or were taken to a place of safety during the incident.
The zoo was shut temporarily during the drama and remained closed until further safety checks were carried out.
Jonnie and Coco had been transferred to Whipsnade from London Zoo about a year ago.
The Captive Animals' Protection Society claimed that this was the latest in a series of animal escapes from UK zoos.
Campaigns manager Craig Redmond said: "In none of these cases has there been any independent inquiries into the escapes."
He added: "How many of these incidents have to happen before local authorities, which license zoos, take serious action to protect the public, zoo staff and animals?"