What Tigermoth said - I also remember how quickly my son could disappear from my side, and seemingly vanish. I remember him disappearing in the supermarket aged about 3, I bent down to pick something up, look round and he'd gone. I assumed he had gone back to the toy aisle and calmly headed back there, no where to be seen. I had several members of staff running around the store with me for about 10 minutes before he was finally located.
The Zoo is solely responsible for this, there is no way a 4 year old should have been able to get where he did.
Once he was in the enclosure and the gorilla had him, I can't see how else the zoo was going to safely extract the child. Gorillas are dangerous animals - zoo keepers do not go in cages with them. He couldn't be tranquilised as it takes time to work and the risk that he would harm the child would be too high. There are experts who read the gorilla's behaviour differently and said he was reacting in a display manner (probably due to all the screams) - where gorilla's will usually drag and throw things around to display there strength. Could they risk him doing that to a 4 year old?
A horrible tragic incident, that ended in the death of a beautiful animal and a probably majorly traumatised child. The Zoo must take full responsibility for this as it failed to ensure the safety of both its visitors and it's animals.