The empress unravels

While her husband was “singing red and smashing black” and her son was living his champagne-fuelled life at Oxford, Gu Kailai's behaviour was becoming increasingly erratic.

In 2005 or 2006, Gu had fallen out with Patrick Devillers, the French architect who had been her main business agent abroad and probably her lover as well. By 2007, with her son at university in the UK, she had returned to China to live with her husband in Chongqing.

In Devillers' wake, Neil Heywood took on a more active role in Gu's financial affairs. Heywood told friends Devillers had been involved in “substantial financial dealings” with Gu and the Frenchman's exit from the scene provided an opening for him to take over. Although they say they cannot rule it out, Heywood's friends had the impression he was not romantically involved with Gu.

In 2008, Heywood began negotiating on Gu's behalf with the British Museum about the possibility of her becoming its “godmother” and patron in China. Described by someone she met during those negotiations as a “very expensively dressed diva” and “very contained, the consummate madam”, Gu entrusted Heywood to manage regular discussions with the museum on her behalf.

“He was an interlocutor, the quintessential western go-between of the kind that is very common representing powerful people in the Middle East,” this person said. But all talks came to a halt in 2010 when Heywood quietly informed people at the Museum that Gu had suffered a nervous breakdown and could not continue with the proposed deal.

Around this time, Heywood began telling friends that Gu was “mentally unstable” and acting like an “empress”. According to a close associate, Gu refused to see most of her old friends from her days as a high-flying lawyer and in photographs taken around this time she appears gaunt and slightly dishevelled.

Instead of going out, she spent most of her time at home in Chongqing doing “handicraft”, according to someone familiar with her illness. She appeared to be suffering from severe depression and according to two people who claim to have knowledge of the matter she may have also been seeking treatment for cancer.

At her trial in August 2012 Gu was almost unrecognizable from her earlier stylish self – she had put on a lot of weight, her face was puffy and she sported a shapeless pudding bowl haircut that looked like it could have been a wig.

Her drastically changed appearance even led to suggestions from some observers that the person who appeared in court was actually a body double but people who knew Gu previously believe it was her and her metamorphosis was the result of chemotherapy or powerful anti-depressants.

During her trial, Gu's government-appointed lawyers presented “expert testimony” from a government facility in Shanghai that concluded she had been suffering from chronic insomnia, anxiety, depression and paranoia.

She had also developed a dependence on “sedative hypnotic drugs which resulted in mental disorders” and she had a “weakened ability to control herself” when she murdered Heywood, according to the government's official verdict.

But the experts also determined that Gu had a clear goal and motive and she was aware of the nature and consequences of her crime. Her paranoia and the frightening power she wielded were on display when she dined out one evening at the Intercontinental Hotel in Chongqing nearly two years before she murdered Heywood.

After flying to Shanghai the next day, Gu came down with an upset stomach and became convinced she was the victim of a poisoning attempt.

Wang Lijun leapt into action and dozens of police descended on the hotel, detained the general manager and most of the chefs and scoured the entire kitchen and supply chain for evidence of poison.

Nothing was found and the Intercontinental employees were eventually released but because of this and similar incidents, hotel and restaurant managers throughout the city were petrified that she would come and eat at their establishment.

On another occasion Gu was dining with Wang Lijun's wife at a Chongqing restaurant when their meal was interrupted by noisy patrons in another room. When they refused to quiet down, the women summoned Wang, who stormed into the restaurant with a police escort and silenced the noisy diners while waving his gun. They turned out to be senior district-level subordinates in the police force and Wang had them fired within days, according to one person with close ties to Wang and the Chongqing police.

According to other people familiar with Gu, her paranoia and depression had been greatly exacerbated by the discovery that she was the subject of a corruption investigation instigated by her husband's numerous political enemies.

Four high-ranking Party members who claim to have knowledge of the matter told the FT that people in the ruling elite who opposed Bo Xilai's bid for promotion to the nine-member Politburo standing committee in the autumn of 2012 had launched a secret investigation to gather evidence on him, his family and associates.

Although background checks and investigations are standard procedure for all Party members expecting a promotion, the breadth and depth of the investigation into Bo was greater than for most other senior cadres, these people say. They say the plan was to confront Bo with damaging allegations in order to block his advancement to the pinnacle of Chinese political power and force him to step down to a less prominent position instead.

These efforts included a probe into Gu's business dealings as well as a prolonged investigation into police chief Wang Lijun, with particular reference to his work in the northeastern mining town of Tieling.

Wang served as deputy police chief and police chief of Tieling in Liaoning province from 1994 until 2003, when he was transferred to serve as police chief of Jinzhou, a larger city in the same province.

In May 2011, Wang Lijun's successor as police chief of Tieling, a 62-year-old Gu Fengjie who also served as vice-mayor of the city, was detained by Communist party anti-corruption investigators.

His detention came not long after two of his deputies, Zhao Tiexing and Fu Xiaodong, had also been detained by anti-corruption authorities. They were later jailed for 14 and 11 years respectively on corruption charges, according to Chinese state media reports.

Provincial officials from Liaoning have denied there is any link between the corruption investigations and Wang Lijun, but senior Party members familiar with the case say they believe these detentions were part of the wider investigation into Wang. In a gruesome twist, the decomposing body of Yuan Weiliang, another vice-mayor from Tieling, was found floating in a canal in the Liaoning provincial capital of Shenyang in September 2011. Police ruled his death a suicide and said he had been depressed.

Friends say Heywood was almost certainly unaware of the investigations into Wang and Gu and the resulting pressure on Gu; he could not have foreseen that his demands and threats would prompt her to take drastic action.

“She probably didn't even think of it as murder,” says a prominent Chinese financier who knew Gu and Bo at an earlier time. “In the Chinese political system people like her are above the law and she probably saw it more as her divine right, something she had to do to protect herself and was sure she would get away with.”