Lee Yoon-hyung was young, beautiful, clever, and rich beyond belief. An heiress to the family that controls South Korea's Samsung Group, she loved fine art and racing cars. She had recently moved to New York and had everything to live for. But then, 11 days ago, she hanged herself in her Manhattan apartment.

It sounds like the classic tale of the poor little rich girl - particularly in the light of reports that 26-year-old Lee had been forbidden by her father, the Samsung chairman, Lee Kun-hee, to marry the man she loved.

But the story of her demise is not quite so straightforward, and its gradual unravelling in recent days has cast a harsh light on the secretive family that controls huge swathes of South Korea's business interests. Best known in the West for its electronics arm, Samsung is the world's largest manufacturer of computer memory chips and is South Korea's biggest and most powerful conglomerate. Lee Kun-hee is one of the richest men in a country that is often dubbed the Republic of Samsung.

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His son and three daughters were well provided for. Lee Yoon-hyung, although not directly involved in the company, had a personal fortune of more than £100m. After graduating in French literature from Ewha Women's University in Seoul last year, she moved to the US to study arts management at New York University. Early last week newspapers in South Korea, typically quoting officials rather than the Samsung family, carried prominent articles announcing her death in a car crash. One official told the press that she had been "pronounced medically dead" following the accident. The South Korean newspaper, JoongAng Daily, reported that she died soon after being taken to hospital, and that a "simple, Buddhist-style" funeral took place in Manhattan on Monday. The Korea Herald quoted yet another Samsung representative saying that the company did not feel obliged to provide more details, adding: "We want to avoid unnecessary rumours that may circulate."

But when the affair was delved further into, it began to look somewhat murky. Three reporters from The Korea Times, a New York-based newspaper, quickly uncovered curious inconsistencies in Samsung's account. It was not clear, for instance, where the accident happened - according to some reports, it took place in the city; according to others, it happened on the outskirts. There were no witnesses, other details were shrouded in vagueness and most crucially, the New York Police Department had no record of a fatal car crash involving Lee.

It wasn't until a week later that the true circumstances of her death were revealed: on Saturday 19 November, Lee hanged herself with an electrical cord in her East Village apartment, by attaching the cord to a door. Her boyfriend, Soobin Shin, and a friend of his, had discovered her body at 3am.

Confronted with this evidence gathered from the police and the city medical examiner's office, Samsung finally confirmed the suicide but simultaneously denied that it had sought to mislead the public. A spokesman in Seoul, Yim Jun-seok, said that, early on, it too had believed that she died in an accident. "At the time the story initially broke, we had an insufficient amount of information," he explained. The company said that, when it learnt the truth, it did not correct the erroneous reports because it regarded the death as a personal matter. "To preserve the dignity of the deceased and out of respect for the bereaved, it was not our place to correct the press reports," the spokesman said. "It was a tragic incident and the family was already suffering."

Few people had any inkling of the acute unhappiness that drove a young woman with a promising future to take her own life. Lee was the most outgoing of the four Samsung heirs, as her elder siblings, her brother, Lee Jae-yong, and two sisters, Lee Pu-jin and Lee Seo-hyun, preferred to keep a low profile. She was one of South Korea's five wealthiest women, with three of the others being her mother and sisters. Her self-confidence and charm were evident in her personal website, called Pretty Yoon Hyung, which since 2003 had detailed her life of privilege. Eventually she was forced to close it down because it grew so popular, but even so her fans created another site devoted to her.

In Korea, she was always accompanied by at least two bodyguards and, after she moved to New York, a driver on 24-hour call escorted her around the city in her limousine. When she enrolled at New York University's Steinhardt School of Education, she took a luxurious apartment in Astor Place, in the bohemian East Village. She apparently hoped that she would eventually take over the running of Samsung's cultural foundation, and had already amassed an impressive personal art collection.

But her glamorous lifestyle, it seems, was a façade. A doorman at her building told reporters that she sometimes stayed in her apartment for a week at a time. It is not clear whether she left a suicide note, but a source close to her suggested that she had been suffering from loneliness and depression during her time in the US. This has been partly attributed to the fact that Lee was pining for a boyfriend in Korea whom she had wished to marry. But her plans were thwarted by her parents, who considered him too lowly a match, and the couple broke up.

Since September, Lee's 64-year-old father had also been in the States receiving post-operative treatment for lung cancer. But although he and Lee's mother, Hong Ra-hee, flew to New York, they did not attend her funeral, as it is customary in Korea for the parents not to attend the funeral of an unmarried child. After Samsung eventually confirmed the suicide, a company spokesman said she had been worried about her father's health and upset by the fact that he was facing a "difficult time" in South Korea.

The secretive way in which Lee Yoon-hyung's death has been handled is said to be typical of a company notorious for its furtive business dealings. Originally founded by her grandfather, in the 1930s, as a small family business exporting fish and vegetables, last year the group's sales were equivalent to one-sixth of the country's annual gross domestic product. But, recently, the company has been embroiled in a string of scandals, ranging from alleged corruption to financial irregularities. Samsung recently pleaded guilty in the US to taking part in a scheme to fix the price of flash-memory chips (sold to Apple at below market prices for use in the iPod Nano music player) and agreed to pay a $300m fine.

Last month two company executives were given suspended jail terms over a deal that helped Lee Kun-hee's children buy a majority stake in an affiliate, through bonds acquired at below market prices in order to transfer corporate control to Lee Yoon-hyung and her siblings. Lee Kun-hee's departure for the US coincided with a request from a parliamentary committee for him to testify about alleged irregularities at a former Samsung automobile unit, and South Korean prosecutors are also investigating allegations that the company provided illegal funds to presidential candidates.

While suicide carries a stigma in South Korea, Yongil Shin, one of the Korea Times reporters assigned to investigate Lee's death, said he was pleased that the truth had emerged. He told The New York Times: "The truth about the death itself, how she died; you owe it to the person, really, to have the correct story out."