An unlocked door allowed two lions to get into an enclosure at Wellington Zoo while their keeper was laying out their food.
When keeper Bob Bennett tripped trying to escape, they mauled him as patrons watched.
Zoo chief executive Alison Lash said the investigation had so far established the lions were not in the enclosure when Mr Bennett, 54, went in to feed them on Tuesday. Standard procedure is to clean the enclosure while the lions are locked up, leave the food behind, exit and lock up before letting the lions in.
"They got in there accidentally, but it's not clear how. But they managed to get the slide [door] open."
Ms Lash said the door was presumably unlocked, because "if it was locked, there's no way they could have got it open ... but that's what we don't know and what we're trying to find out".
She could not comment on whose responsibility it was to ensure the slide was locked but said the investigation would cover that aspect.
"Mr Bennett was exiting when he saw the lions. It's pretty clear he would have stood a reasonable chance of getting away unharmed except that he tripped on the way out and that appeared to trigger the lions' chase instinct."
Mr Bennett was not giving interviews at Wellington Hospital yesterday but was said to be shocked and sore. He was to undergo minor surgery so doctors could clean his wounds and ascertain whether one wound caused internal damage to a joint.
The incident was captured on video by the Wang family of Tauranga. Sam Wang said he immediately called the zoo reception on his cellphone while other family members shouted to distract the young male lions, Malik and Zulu.
His father-in-law continued videotaping.
Zoo general manager operations Mauritz Basson said the ordeal, which lasted 15 to 20 minutes, could not be classified as an "attack" and the cats were only playing.
"The lions did not attack Bob. They were more interested in the buckets he had in his hand and unfortunately he tripped over a bush and fell and that's when the lions decided 'Oh, this is a handy toy'."
If the lions, each weighing about 130kg, really wanted to attack, "they would have killed Bob in a matter of seconds".
"They don't waste time, and we are very, very flimsy creatures compared to a lion."
The Wang family sold their footage to TV3 for $1000. The money will be given to Mr Bennett and the Cancer Society.