Back in April, a team of veterinary specialists flew to Malaysia to perform urgent dental surgery on Puntung, one of the country’s last three Sumatran rhinos. The procedure to correct Puntung’s life-threatening abscess seemed to be a success: Within hours of the operation, the rhino was chomping on tasty foliage. The effort to save her captured attention across the globe.

But as Christina Nunez reports for National Geographic, experts later determined that Puntung’s abscess was a symptom of cancer, and her condition began to decline. To alleviate her suffering, Puntung was euthanized on Sunday.

“Today is one of the saddest days we've ever faced,” the Borneo Rhino Alliance, which had been caring for Puntung, wrote in a Facebook post. The organization went on to explain that while it had considered performing chemotherapy, radiation, and further surgeries, those treatments would likely have caused Puntung further distress.

“Sumatran rhinos wallow in mud for at least six hours daily and become increasingly stressed if kept in clean, closed facilities,” the post says. “A stress-free life for Puntung was simply not going to be possible.”

When Puntung first made headlines in April, she became a symbol of hope in the fight to save critically endangered species. Poachers and hunters seek rhino horn due to the false belief that it has medicinal value. There are less than 100 Sumatran rhinos in the world today and, with Puntung’s death, only two are now left in Malaysia.

As Austa Somvichian-Clausen reported for National Geographic earlier this year, efforts to treat Puntung began when South Africa-based journalist Adam Welz tweeted about the 20-year-old rhino’s plight. Welz then contacted Saving the Survivors, a South African non-profit that provides care to endangered animals. Plans were made to fly specialists from Singapore and Thailand to the Tabin Wildlife Reserve in Sabah, Malayasia so they could perform surgery on Puntung.

Several weeks after the operation, the Borneo Rhino Alliance announced that the swelling on Puntung’s left check, which had alerted the organization to her infected tooth root, “had a more serious origin.” Her cancer was spreading rapidly, leaving her in pain and unable to breathe through her left nostril.

Nancy Lai of the Borneo Post reports that Puntung’s keepers slept with the rhino in her forest paddock so she would be under constant supervision. They noted that Puntung would periodically bleed from her nostrils.

“In consultation with our rhino reproduction advisers at Leibniz Institute for Zoo and Wildlife Research in Berlin, and others, the decision was taken to end her growing discomfort,” Augustine Tuuga, department director of Sabah Wildlife, said in a statement, according to Lai.

Tuuga also noted that a reproductive specialist had been flown in from Jakarta to recover Puntung’s egg cells, which might help conservationists breed more Sumatran rhinos in the future.

The Borneo Rhino Alliance writes on Facebook that Puntung's life was far from easy. "She survived a poacher’s attempt as a calf, when her foot was cut off. But she refused to give up and went on to survive in the forests," they write. "We’ll always remember her as a fighter."