BERLIN – Vets began Monday an autopsy on Berlin Zoo's superstar polar bear Knut after his untimely and unexplained death aged just four, as fans left flowers, candles and poignant messages at his former home.

However, it remained unclear when the results from the examination of Knut, who was found lifeless in the pool in his enclosure on Saturday afternoon, would be published.

"I cannot give you a timeline. We will inform the public when we know what the exact cause was," spokeswoman Claudia Bienek told AFP.

Knut first shot to fame as an uber-cute cub after being abandoned by his mother and reared by hand. His first public appearance in March 2007 drew around 100 television camera crews from around the world.

Devastated fans on Monday left bouquets of pink and red roses, tulips, messages of love, photos of Knut and even a croissant to mark their beloved's passing at the zoo.

"You left us completely unexpectedly, at a moment when televisions the world over are showing images of war and catastrophe," read one message from a fan who added that Knut "brought such joy and harmony in the world".

Bienek said there were currently no plans to erect a memorial to Knut, adding that a "book of condolence" had already been set up for fans to record their memories of the bear.

"I've been blubbering for days," said one entry of the 1,700 left online.

"I am in complete despair over the death of our polar bear Knut. I cannot put my grief in words," wrote another fan.

Once Knut grew into a strapping adolescent and then adult, animal welfare groups began to worry that he was displaying abnormal behaviour because of all the attention.

He would sway to and fro and even imitate people taking photos by lifting a paw to his face. Hopes that Knut might form a romantic attachment with one of the females were dashed as stories emerged of violent maulings.