In the dense, hilly jungles of southwest Vietnam, a lone rhino once wandered. She was the last of her subspecies and this was her home.

Cat Loc, a northern sector of Cat Tien National Park, is a part of the world once ravaged by Agent Orange during the Vietnam War. Today it is better known as a wildlife conservation area – but also a place where some of those efforts have failed.

The last rhino spent her days roaming across thousands of hectares, a much wider range than was thought natural for these herbivores. But then again, she had the run of the place. There were creeks and rivers where she could wallow and there was also plenty of food – like rattan, a woody climbing plant found all over the area.

But one day, a hunter peered at her through the sights of a semi-automatic weapon – and pulled the trigger. We do not know if the rhino saw her killer and we do not know how many times she was shot. But as that gunshot cracked out in echoes across the forest, the extinction of Javan rhinos in Vietnam was sealed. However, it did not happen immediately. The rhino, though wounded, managed to escape. And so, for a time, she disappeared into the thick greenery that sustained her.

The fortunes of Javan rhinos in Vietnam, a subspecies called Rhinoceros sondaicus annamiticus, were closely followed at the time by Sarah Brook. Brook was working then as a conservationist for the World Wide Fund for Nature (WWF). Her days were spent struggling through the inhospitable terrain of Cat Loc – steep hills and barely penetrable vegetation – with a sniffer dog, trained to follow the scent of rhino dung.

One day, a hunter peered at her through the sights of a semi-automatic weapon – and pulled the trigger

For nearly six months Brook had been collecting dung samples, on the trail of Javan rhinos in the region, not knowing for sure that there was only one. She worked closely with the national park rangers and set video camera traps.

But the traps were set too late. They never captured footage of the rhino and in all that time, Brook never once saw it alive with her own eyes. The Cat Loc forest is big, and, as it turned out, this rhino was all alone inside it.

In early 2010, distressing news arrived.

A ranger sent Brook photographs of a rhino skeleton, discovered in April. A photo of the skull, separated from the rest of the animal's bones, clearly showed that the horn had been removed. In fact, it had been crudely hacked off. Poaching.

A crack team of experts from around the world found themselves assembled at the national park

The park had already issued a statement saying that the rhino's death was the result of natural causes, but Brook was not so sure. And, upon checking the bones a few weeks later in May, she discovered a bullet lodged in the left foreleg.

"We wanted to find out what had actually happened and get the truth out," she remembers. "By that time, we already suspected that was actually the last individual."

Brook swung into her investigation and began by trying to find the right people to help.

She fired off an email to Ulrike Streicher, a wildlife veterinarian who had been living in Vietnam for years. She knew Cat Tien well and had consulted for the WWF before.

They needed to visit the place where the rhino's skeleton had been found

Brook also contacted Ed Newcomer, a special agent from the US Fish and Wildlife Service. He was based in Thailand at the time but leapt at the chance to be involved with a rhino investigation.

John Cooper, a wildlife forensics expert from the UK, and Douglas McCarty, who worked for the anti-wildlife trafficking group Freeland, were also enlisted.

By September, a crack team of experts from around the world found themselves assembled at the national park, in a room filled with biological artefacts. There were display cases and cabinets with everything from casts of rhino footprints to maps of the area, recalls Newcomer. They did not have a specialised veterinary area to work in, but this would have to do.

The first task was to take a look at the recovered bones.

As one of the rangers presented the bones to the team, Cooper remarked that they looked as though they had been bleached.

It was extremely rugged

"They said, 'oh we bleached those', and I said, 'OK, well there goes the potential to take any samples'," says Streicher. "The only people that dealt with wild animal carcasses in Vietnam were the ones who worked in museums – and their advice would have been to bleach the bones."

Without the possibility of taking samples from the bones for analysis, the job of figuring out a cause of death instantly became much harder. Everyone concurred that they needed to visit the place where the rhino's skeleton had been found. That meant a seven-kilometre trek into thick jungle – and seven kilometres back again.

The team made their way across this terrain on a hot and very humid day. A guide, paced out in front of them, macheted a path through the branches and vines. "It was extremely rugged," remembers Newcomer.

It was Newcomer whose insights were key for this part of the investigation.

Eventually, the ravine where the rhino had been found appeared before them. There was a very steep, heavily forested incline to the north and a shallower incline to the south. Below was a small creek flowing roughly from east to west.

"It was incredibly moving," he recalls. "It hits you like a brick."

"There were still rhino tracks in the dirt," says Newcomer. "And above where the carcass had been found there were these very robust bamboo trees – they were maybe four to six inches [10-15cm] in diameter – and they had been completely pushed over at the base, like a bulldozer had pushed them over."

Newcomer saw one length of bamboo that had broken in a U-shape. Everyone instantly imagined the rhino lying in it, perhaps after falling and getting entangled.

"If it had its belly hanging over those bamboo trees, it probably suffocated," says Newcomer.

Gradually a picture of the death was beginning to emerge. This rhino had been moving around before it died, it had struggled, and it had interacted with the vegetation.

They checked the site for any bones that had not been collected, but found none. Newcomer made some quick sketches of the area, noting the location of key features like the tracks and the spot where the skeleton had been found, relative to the creek.

As they left the site, Newcomer – the last to leave – turned round to take a picture. He knew he would not be back in person. The possibility that this really was the spot where the Javan rhino in Vietnam finally died out struck him at that moment. He waited a few seconds, looking at the crushed bamboo and the creek below.

"It was incredibly moving," he recalls. "It hits you like a brick."

The rhino did not just die a few days or weeks after being shot. It had taken months

The team hiked back through the jungle. In the afternoon it started to rain, and some of the trek was right through a creek. By the end, they were soaked through and exhausted.

But back at base during the following days, the next task was to reassemble the rhino's skeleton on the floor as accurately as possible. A closer look at the bones could reveal a lot about the animal's health just prior to its death.

Cooper and Streicher were experts in this sort of work. Among other features, they noticed that there were arthritic changes in the left foreleg, which would have caused stiffness and probably pain. There was also less wear on the hoof of this leg, suggesting that it had not been functioning normally.

Cooper has written about this examination in a recent book, Wildlife Forensic Investigation: "It was probably … that the injuries that the bullet caused, culminating in a stiff, painful, arthritic left forelimb, predisposed the rhino to other potentially hazardous, possibly fatal, factors."

Later, Streicher took this particular bone for a computerised tomography (CT) scan. It showed a cavity below and above where the bullet stopped, likely caused by a resulting process of inflammation – osteitis. In a report for the WWF, the investigators noted that this process would have taken at least two months, perhaps as many as five.

Here was a key clue. The rhino did not just die a few days or weeks after being shot. It had taken months. And it was not just wounded for a short time, but injured in a way that impeded its movement and eventually, it seemed, led to its death.

"Animals like this – rhinos, elephants – they can't do without a leg," explains Streicher. "They're too heavy to limp."

Although rhino horn can fetch many thousands of dollars, the horns of Javan rhinos are not very large

Newcomer also noted that, given the entry point of the bullet and its apparent trajectory, it was most likely fired from a poacher who had been positioned below the rhino – perhaps on a hillside – and who was probably aiming for the animal's heart.

It is possible more than one shot struck the creature, but any embedded in or fired through soft tissue rather than driven into bone would not have been found by the team. All they had was the skeleton.

Another unsolved question, then, is that of whether it was the shooter who tracked the injured rhino for months in order to take off its horn – or an opportunistic scavenger who came across the carcass later.

Either way, although rhino horn can fetch many thousands of dollars, the horns of Javan rhinos are not very large. "It easily fits into the palm of your hand – you could fit it in your pocket," says Newcomer as he remarks on the miserable circumstances – a subspecies cast into extinction for the sake of a tiny horn.

A few months later, DNA analysis confirmed that the dung collected earlier by Brook all came from the same animal – and that that animal was indeed the one whose skeleton had been examined.

With so little evidence to go on, it is difficult to be conclusive, but the signs certainly point to the following sequence of events: a poacher shot the rhino at least once; a bullet lodged in the left foreleg caused inflammation, arthritis and reduced mobility; this may or may not have led directly to a fall in the forest near a creek, where the animal finally succumbed.

Whoever shot the rhino and whoever took its horn – whether or not they are the same person – likely remains at large

"The gunshot did kill the rhino," declares Newcomer. "It just took a long time to do it."

Brook later described what happened as a "conservation failure" for Vietnam. "I was quite affected by it, and actually quite depressed afterwards," she recalls. Her main reason for being in Vietnam was to survey the rhino population, and that population was now gone.

The Vietnamese police assisted with the investigation, providing ballistics analysis of the bullet. This suggested it came from a semi-automatic weapon, such as an AK47.

But none of the investigators who spoke to the BBC are aware of any subsequent arrests or convictions. The WWF has said it believes an official investigation remains open. Whoever shot the rhino and whoever took its horn – whether or not they are the same person – likely remains at large.

A few Javan rhinos still exist in the world. There is a small population – between 40 and 60 individuals – in Indonesia. But they are listed as critically endangered by the International Union for Conservation of Nature (IUCN). Patrols organised by the International Rhino Foundation and the Rhino Foundation of Indonesia keep a watch on the population, but these groups warn that the size of the habitat available to the rhinos should be expanded to ensure their continued survival.

A ban on international ivory sales has helped to slow, though not halt, the poaching of elephants

Today, Ed Newcomer is stationed in Botswana. He is still very much concerned with the plight of rhinos. Nearby, in South Africa's Kruger National Park, three or four rhinos are poached every day. And there is a bitter irony to the killings.

"Those rhinos are being killed to fuel the demand in Vietnam," he says.

But there is hope. For one thing, as Newcomer points out, humans have been successful at combating poaching before. A ban on international ivory sales has helped to slow, though not halt, the poaching of elephants, for example. A similar ban is in place with regard to rhino horn, though there are still many who challenge local laws.

Brook, who now works for the Wildlife Conservation Society (WCS), notes that for many conservationists in Vietnam, the loss of their last Javan rhino was a profound shock. It may be too late to save that subspecies, but there have since been efforts to focus attention on other endangered rarities.

She gives the example of the saola, one of the rarest large mammals in the world. Resembling a small antelope, the saola was only discovered in 1992 and is endemic to Vietnam and Laos.

The loss of this rhino was a tragedy for Cat Tien, but the park's mission is not over yet

"They basically changed their approach to conservation of that species in central Vietnam and established a co-management arrangement with these new protected areas – primarily to protect the saola," explains Brook. "That project has been incredibly successful in terms of removing massive numbers of snares from the landscape."

The horn of the last Javan rhino in Vietnam in all likelihood vanished into the black market. Even if it were found, it couldn't in this case be genetically confirmed as belonging to the examined individual. The perpetrators of this poaching event will probably never be caught.

And yet, in the jungles of southeast Asia, many wonderful creatures still survive – and many flourish. The loss of this rhino was a tragedy for Cat Tien, but the park's mission is not over yet. Right across its 720 sq km, macaques, sun bears, pangolin and rare deer just about pull through. With any luck, they will never cross the poachers' sights out there, deep in the vastness of the forest.

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