The man who hacked off nine-year-old Fortune Khumalo's genitals struck as the youngster relieved himself in bushes. Using a machete, the attacker sliced off Fortune's penis and testicles, to sell the body parts to the lucrative traditional medicine - muti- market.

Fortune had become a victim of a crime of the occult that has rocketed in recent years to supply a booming trade in human body parts. He survived the attack but most victims do not and the Government estimates that there could be more than 300 muti murders a year. “The killings are driven by greed,” said Thomas Khumalo, Fortune's father. “People believe using human body parts in medicine can make them rich.”

Fortune's assailant was caught with the genitals wrapped in a towel in his pocket. He planned to sell them to a traditional healer in Johannesburg, where body parts are worth up to £250, and a human head can go for as much as £500.

Fortune is now in constant pain because his family cannot afford the reconstructive surgery he needs.

According to Special Superintendent Gerard Labuschagne, an expert on ritual killings, rogue traditional healers, known as sangomas, are behind the muti murders, prescribing body parts to patients and then hiring men to carry out the killings. Belief in the power of sangomas runs deep and many black South Africans visit them for anything from health matters to spiritual guidance.

Human genitals are the most prized parts and can be used to attract wealth and increase fertility. Children's body parts are believed to be the most potent. They are cooked and ground down, to be used with herbs and other ingredients. Sometimes parts are used whole - it is believed that if a human arm is waved around each morning in commercial premises it will draw customers.

Most muti murders have been concentrated in the poor rural province of Limpopo, in the north of the country, but recently the phenomenon has been spreading. In the town of Bizana, in Eastern Cape province, 18 people were murdered by a muti gang in nine months. Police arrested 14 men, but locals believe the sangomas behind the attacks were never caught. Vigilante groups now patrol the area at night.

“People are really scared. Even with a police presence here, the killings continued so we decided we had to act to protect ourselves,” said Bongani Danga, the head of the vigilantes. The group has set up a night-time curfew and anyone breaking it is stopped and searched for machetes and hammers, the instruments used in attacks.

The Yalo family were the last victims; a number of men had been arrested and 17-year-old Anelisa thought the area was safe. But after spending the night at a friend's house she returned home to find the butchered body of her two sisters and her father.

“There was blood everywhere,” she said. “They had taken my sisters' breasts and genitals.” Victims are often tortured as they are killed as sangomas believe that the muti is more powerful if the person is alive and in pain when their parts are taken.

Many muti murders are not reported, through fear. Even when they are, they are not always investigated properly. “An investigative officer tasked to deal with this might be a bit hesitant because of his own traditional beliefs,” said Superintendent Labuschagne, who visited Britain seven years ago to help to investigate the still-unsolved mystery of “Adam”, the torso of a boy found in the Thames.

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