Image copyright AFP/Getty Images Image caption More than four years on, Vorayuth Yoovidhaya has not appeared in court

Among the many uncertainties hanging over Thailand as it is steered hesitantly through a delicate political transition, its people could be sure of one thing this week.

That Vorayuth Yoovidhaya, the grandson of the billionaire who invented the energy drink Red Bull, would once again fail to appear at a Bangkok prosecutor's office to face charges relating to the death of a police officer.

On 3 September 2012, that officer was struck by a Ferrari driven by Mr Vorayuth.

The sometimes farcical attempts by the Thai authorities to bring Mr Vorayuth to justice are now commonly cited as epitomising the untouchability of the super-rich in Thailand.

The facts of the incident early on the morning of 3 September are reasonably clear.

Police Sergeant-Major Wichian Klanprasert was riding his motorbike along Bangkok's Sukhumvit Road when he was hit by a grey Ferrari, which dragged his body more than 100m (109yds) down the road, before driving off.

Investigating officers followed a trail of brake fluid to a luxury home less than a kilometre away, owned by one of Thailand's wealthiest families.

The badly-dented Ferrari was there, but initially the police were persuaded to detain a driver employed by the family as their main suspect.

When they subsequently discovered the car had actually been driven by Mr Vorayuth, then 27 years old, he was tested and found to have excessive alcohol in his blood - but, he said, this was from drinking at home after the accident.

Image copyright Reuters Image caption The badly-dented Ferrari was found in the compound of the family's home

The police believe from security camera videos, the distance the car travelled after the crash, and the injuries that instantly killed Sergeant-Major Wichian, that Mr Vorayuth must have been speeding, they estimate at around 170km/h (106mph) in an 80km/h zone. His lawyers have denied this.

It took the police six months to prepare criminal charges of speeding, reckless driving causing death, and fleeing the scene of an accident.

Throughout 2013, Mr Vorayuth failed to appear seven times to hear the charges, with his lawyers providing an array of justifications, from him being on business overseas to feeling unwell.

In September 2013 the limitation period for the speeding charge expired.

Global success

Mr Vorayuth's grandfather, Chaleo Yoovidhya, made his fortune when, in the mid-1980s, he teamed up with an Austrian marketing executive to turn his energy drink known as Kratindaeng, or Red Bull, into a global sales success.

Today the extended Yoovidhya family is believed to be worth more than $20bn (£16bn). The Red Bull logo is now seen all over the world, in particular sponsoring spectacular stunts and sporting events.

Image copyright AFP/Getty Images Image caption Red Bull sponsors a Formula One team, as well as teams in other motorsports

But the family has shunned the limelight; before his death in March 2012 Chaleo Yoovidhya never gave a single media interview. After the accident, his grandson, Mr Vorayuth, vanished from public view.

But social media posts by him and his jet-setting friends suggest he has often been inside Thailand, as well as travelling around the world to motor racing competitions or beach resorts.

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Right after the accident the police chief of Bangkok at that time, Kamronwit Thoopkrajang, promised the public that the culprit in Sergeant-Major Wichian's death would be brought to justice, or he would resign.

In April 2013 the attorney-general promised to indict Mr Vorayuth, only to backtrack after he was petitioned by the suspect's lawyers, who contested the fairness of the speeding charge.

In September 2013 the prosecutor ordered police to arrest him after his seventh no-show. Nothing happened.

Then, against a backdrop of growing political turmoil in Thailand, the case appeared to be forgotten.

Public interest in it was revived only after a horrifying road accident last year, involving another wealthy young man who drove his luxury car at high speed into another vehicle, killing two graduate students.

People started asking what had happened to the Red Bull heir. And a military government, which had promised to address the abuses of previous governments, felt forced to act. Or, perhaps more accurately, to be seen to be acting.

In March last year the Attorney-General announced that he would once again press charges against Mr Vorayuth.

But throughout last year, his lawyers successfully postponed repeated requests for him to report to the prosecutor's office, claiming that their client had filed a complaint of unfair treatment to the National Legislative Assembly, the military-appointed parliament.

Lawyers spoken to by the BBC say there is no legal justification for using this device to delay proceedings against Mr Vorayuth, but that is what is happening.

'On business in the UK'

Today the police insist they can do nothing.

Asked why they have not issued an arrest warrant against the accused, as requested three and a half years ago, they told the BBC that it is up to the Attorney-General's office to act.

The Attorney-General's office says he cannot be indicted unless he appears in person.

And for the latest request for him to report to the prosecutor's office to hear those charges? Mr Vorayuth, we are now told by his lawyers, is on business in the UK. The Attorney-General has once again granted a postponement, to next month.

Image copyright Getty Images Image caption The extended Yoovidhya family is believed to be worth more than $20bn (£16bn) thanks to their Red Bull empire

The relatives of Sergeant-Major Wichian have said little about the case. As usually happens in these situations, the Yoovidhya family have paid them a large sum of money, around $100,000.

In return they have agreed not to press charges themselves.

And public interest in Thailand will probably wane, as people wearily conclude that, once again, the rich have shown that they are beyond the reach of the law, in a country ridden with corruption and abuses of power.

One lawyer, who was once closely connected to the case, told the BBC he had never seen another example of a suspect evading justice like this one.

Had it been someone else, without a powerful family behind him, he said, he would certainly have been arrested the first time he failed to report himself.

The most serious charge against Mr Vorayuth, of reckless driving causing death, expires in the year 2027.

Few people are betting that he will face any legal sanction, or indeed any meaningful restrictions on his lifestyle before that deadline frees him completely from any lingering repercussions of the events of that morning four and a half years ago.

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