Zoo worker Sarah McClay died after being attacked by a Sumatran tiger Friday in an enclosure at South Lakes Wild Animal Park near Dalton-in-Furness, Cumbria.
Police said McClay's death could have been due to "human or technical factors," but are still investigating how the 24-year-old came into contact with the animal.
Cumbria Police Detective Superintendent Andrew Slattery said the investigation was centered on "the management of dangerous animals."
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He said "systems and protocols" at the park were being investigated, but that it was not being considered a criminal investigation.
According to police, compartments in the tiger enclosure were connected by lockable doors, but systems put in place to keep staff and animals apart had "failed."
Park owner David Gill said there was "no reason" for McClay to be in the tiger enclosure.
He asked for the public's support, understanding and prayers.
"The park is a very safe environment and never at any stage were any other public or staff in danger," Gill posted Saturday on the zoo's Facebook page. "We have been given the all clear by the authorities after initial investigations yesterday and we hope you will allow us all to work through this tragic event."
Gill said McClay was originally from Glasgow and was a "very experienced" member of his staff who was "proficient and passionate" at her job.
"We have very strict protocols and procedures for working with big cats, but it seems she failed to follow correct procedures," he said.
"For inexplicable reasons she opened a door and walked into the enclosure.
"We will never know why she entered without telling anyone. There was no reason for her to go in there."