Investigators comb the crime scene, planting a small red flag in the spot where a bullet shell lies near the latest victim in South Africa's Kruger National Park.

Vultures circle above the gigantic decomposing rhino carcass baking in the sun in the sparse bush after illegal poachers shot it for its horn.

Almost 900 rhinos have been killed this year alone in South Africa.

A hole gapes above the bull's nose, where a poacher hacked off its distinctive horn for the Asian black market. Scavengers have already feasted on the wound.

The horn of his female companion, lying 100 metres (yards) away, is intact, suggesting the poachers may have been disturbed before finishing the job and fleeing across the border to Mozambique, just four kilometres (2.5 miles) away.

South African wildlife authorities search for all the clues they can to help establish what strategy the criminals use and any pattern to the killings.

"We collect every piece of evidence left behind by the poachers, including cartridges used to shoot the animal," said Kobus de Wet, chief environmental crime investigator at South African National Parks.

"The evidence is presented in court, if we happen to link any person to the crime."

Investigators wearing elbow-length rubber gloves scan the bull's thick hide with a metal detector, then cut the area around its wound open with scalpels for their postmortem report.

Evoking the American television drama series "CSI: Crime Scene Investigation", the wildlife sleuths place bullet fragments, casings and other pieces of evidence in small plastic bags for forensic processing.

The experts conclude that two shots from a .458 calibre hunting rifle finished off the bull. But they are not yet sure what killed the female.

"The poachers use powerful hunting weapons and have the skills of professional hunters," De Wet said.

Depleting numbers

South Africa is home to around 80 percent of the world's rhino population, estimated at more than 25,000.

Most dwell in the vast Kruger Park -- roughly the size of Israel -- which is also the poachers' preferred hunting territory.

More than 60 percent of South Africa's 891 rhino poachings since January were in Kruger.

Some animals are darted with tranquilisers and hacked alive in the dead of night.

Not even young calves with their mothers are spared.

Park officials say most poachers come from Mozambique's impoverished villages, recruited by international syndicates.

A next level of operatives then smuggles the valuable horns to Asia. A kilogramme of rhino horn can fetch around $65,000 (48,000 euros), according to South African environmental authorities.

The trade is driven by demand for the horns in China, Vietnam and Laos, where owning even parts of a horn is seen as a status symbol.

Made of the same material as human fingernails, the horn is often ground into powder and consumed in the mistaken belief that it will bring special powers.

Over the past four years the killings have depleted the park's rhino population to below 10,000.

In a crackdown on poachers, more than 450 rangers at Kruger have been transformed into counter-insurgency paramilitary troopers, conducting park-wide patrols using five helicopters that help them get around the vast space. In addition, soldiers patrol the border area.

Authorities have arrested nearly 300 poachers since the start of the year, already more than the 267 detained in 2012, though very few cases are successfully prosecuted, and in most instances they drag on for a long time.

But those arrested are often low-level hunters who simply carry out orders; the kingpins who arrange the transport of the horns halfway around the world are rarely caught.

The hunt for those culprits continues.