When a female Sumatran rhino was captured in a pit trap in March in the Indonesian part of Borneo, the conservation community cheered. The endangered species had not been physically encountered in the area for 40 years. But now that hopeful story has taken a sad turn, as the rhino—known as Najaq—has died.

"Our hearts are saddened by this devastating news from Kalimantan," the International Rhino Foundation wrote on its Facebook page Tuesday.

Najaq succumbed to a leg infection after her health deteriorated over the past few days, Indonesia's environment minister said. Further investigations are pending.

According to Arnold Sitompul, conservation director of WWF-Indonesia, early evidence suggest the infection was first caused by a snare from an earlier poaching attempt, before the animal was caught by conservationists.

"The sad death of this rhino reminds us of the tremendous challenges associated with protecting the Sumatran rhino population in the Indonesian part of Borneo," says Sitompul.

Najaq was thought to be four or five years old. She was captured in a pit trap in Kutai Barat in East Kalimantan on March 12. Sumatran rhinos had been thought extinct from Kalimantan, the Indonesian part of Borneo, but some camera trap evidence gathered in 2013 had suggested the return of at least a few individuals in the past few years.

As a result of that evidence, conservationists had set a trap with the goal of moving any rhinos from the area to a more secure sanctuary, about 93 miles (150 kilometers) away. The region was too close to mining operations and plantations to be considered safe habitat.

View Images Najaq is one of only about 100 Sumatran rhinos left. Photograph by Ari WIBOWO, WWF-Indonesia

"It is our hope that the next rhino captured in Kalimantan will be sent to the Sumatran Rhino Sanctuary where it can be cared for in a permanent facility by experienced veterinarians and keepers," the International Rhino Foundation wrote.

"Most importantly, we hope that the next rhino captured will be part of the much-needed Sumatran rhino metapopulation management strategy, while concurrent surveys are conducted to accurately determine the population in Kalimantan and appropriate long-term plans made."

The smallest of the world's five species of rhinos, the Sumatran is covered with patches of stiff hair. Dark red-brown in color, the animals prefer to live in dense mountain forests, where they are highly elusive. They are usually solitary creatures that feed on fruit, twigs, and leaves. They can find one another by leaving scent trails, which they can pick up with their keen sense of smell. (Learn more about efforts to save the species.)

Sumatran rhinos can grow up to 1,760 pounds (800 kilograms) and reach a length of 8 to 10 feet (2.5 to 3.2 meters). About 100 Sumatran rhinos are thought to exist in the wild, mostly on the island of Sumatra, making them one of the rarest mammals on the planet.

While their two horns are considerably smaller than those on African rhinos, the appendages are still prized on the black market for purported health benefits, despite scientific evidence proving that such treatments don't work. Their population has been hammered over the past century by poaching and loss of habitat from deforestation, mining, and other impacts. The rhino was declared extinct from the Malaysian section of Borneo last year.

"This [death] demonstrates the threats faced by the Sumatran rhino and underscores why we need to continue our efforts with the strong support of the government and other experts to save the remaining population of Sumatran rhinos in the area," says Sitompul.

This story was updated at 4:17 pm to include additional info from WWF.