Clyde the turtle was taken from the Blue Planet Aquarium in Ellesmere Port, Cheshire, on Tuesday.
They also revealed that a young boy had managed to get into the tank and remove the creature at around 5.30pm.
A young boy with no top on was spotted in Clyde's tank at Blue Planet Aquarium in Ellesmere Port on Tuesday afternoon, Cheshire police said. The boy was completely soaked and had no top on, which may have attracted your attention if you had been there yesterday.
Staff at the aquarium say the turtle is likely to die without their specialist care.
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CCTV images appear to show the boy carrying something under his jumper, which police believe was the turtle.
Hundreds of people reacted with dismay after police broke news of the theft yesterday.
Without specialist care, Clyde may die, he added.
Anyone who recognises the woman in the CCTV or has any information that may lead to the safe return of Clyde is urged to call 101 quoting incident number 719 of 30 August.
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"Did you see anyone enter any of the enclosures or perhaps in sodden clothing?"
The force said it was keeping an "open mind" of the circumstances of the theft.
He was then seen walking away carrying something under a jumper, which detectives believe was Clyde, a 20cm mud turtle.
The underwater venue is visited by thousands of people from Greater Manchester each year.
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Information can be passed anonymously by contacting Crimestoppers on 0800 555 111.