Three newborn polar bears died recently from unknown causes just days after birth at Toronto Zoo, according to Zoo officials. Their cause of death is being investigated but it is known that their mother killed two of her cubs from the last litter.

On Monday, officials at Toronto Zoo mourned the loss of three polar cubs, one female and two males. The baby bears were born last Thursday and died on Sunday.

The zoo is still unsure of how the polar bears died. They said the cubs were "being mothered, nurtured and nursed" by their mother, an 11-year-old polar bear named Aurora.

"All was going well," said zoo officials, who had been monitoring the den around the clock.

Last year, Aurora killed two of her three cubs. Only one cub named Hudson survived the violent attack by the mother. The death of the new litter comes just one week after Hudson celebrated his first birthday.

Tissue samples from the recently deceased polar bear cubs are being sent for analysis.

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Despite the tragic loss, the zoo called the experience a step in "a positive direction" for the breeding program with Aurora.

"She was learning to be a mother," said zoo officials. All the zoo staff are reportedly disappointed by the death of the cubs.

Maria Franke, the zoo's curator of mammals, defended Aurora, saying there are "many possibilities" of what could have happened to cause the newborn polar bears to die.

"Little is known about polar bear births and rearing in the wild and it is possible that young female bears gain experience over time before successfully raising their young," Franke stated.

Polar bears are an endangered species. A majority of the polar bear population on Earth resides in Canada.

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