On 2 October at 13:14, Jamal Khashoggi walked into a nondescript beige building in the Levent area of Istanbul.

A prominent writer and outspoken critic of MBS, he was only visiting the Saudi consulate to get some divorce papers certified.

But once inside he was overpowered by a hit team of security and intelligence agents sent from Riyadh, murdered, dismembered and his body disposed of, never to be found.

Jamal Khashoggi Jamal Khashoggi

Thousands have died in the war that has devastated Yemen, many through Saudi-led airstrikes. Hundreds of Saudis critical of MBS’s policies have vanished into prisons. And yet it took this particularly gruesome murder of a journalist to turn much of the world against the Saudi crown prince.

Despite official Saudi denials, Western intelligence agencies believe that at the very least it was highly likely MBS knew in advance about the operation to silence Khashoggi. According to reports, the CIA believes he actually ordered it.

In an interview aired on 29 September with CBS 60 Minutes, the crown prince went so far as to “take full responsibility” for what happened. In an earlier PBS interview he said it happened “under his watch”. Yet this is not the same as accepting culpability, which he and his government still deny.

A key link in this whole, hideous murder mystery is one of MBS’s closest former advisers, a 41-year-old ex-air force man, Saud al-Qahtani.

Until he was dismissed from his job soon after the Khashoggi murder- on the orders of King Salman - he was the crown prince’s unofficial gatekeeper in the all-powerful royal court.

Al-Qahtani is said to have managed an aggressive cyber surveillance strategy that monitored Saudis both at home and abroad, using intrusive software programmes. According to some reports, this included turning their mobile phones into bugging devices without the owner knowing.

Anyone critical of MBS or his policies found themselves trolled with abusive and threatening messages on social media. With over a million Twitter followers, al-Qahtani was able to mobilise an “army of flies” to harass and shame perceived enemies.

Saud al-Qahtani's Twitter profile image Saud al-Qahtani's Twitter profile image

By the summer of 2017, with Saudi bloggers, democracy campaigners, human rights activists and others getting rounded up and imprisoned. Jamal Khashoggi knew he too was in danger.

In June that year, the same month that MBS became crown prince, Khashoggi got out of Saudi Arabia while he could, flying into exile in the US.

The 59-year-old journalist had always called himself a Saudi patriot - he used to be the media adviser to the Saudi ambassador to London in the early 2000s and I would go and have coffee with him in those days.

But after moving to the US he wrote Washington Post columns that grew increasingly critical of MBS’s autocratic style. The crown prince is said to have become irritated.

The journalist started to receive calls from Riyadh urging him to return, promising him safe passage and even a job in government.

Khashoggi didn’t trust these assurances. He told friends that al-Qahtani’s team had hacked into his emails and text messages and read conversations with other dissenting figures.

Khashoggi and others had a plan to launch a movement for free speech in the Arab world. He had 1.6 million Twitter followers and was one of the Middle East’s most prominent journalists.

To MBS and his close advisers, Khashoggi would have been seen as a clear and present threat, although again, the crown prince denied this in his CBS interview.

Historically, there have been several cases of the Saudi leadership abducting a number of “wayward” citizens, even princes, and taking them back to Riyadh to “bring them back into line”, as commentators have put it. But not murder. An assassination in a foreign city would have been a drastic departure from their normal modus operandi.

The door to the Saudi consulate in Istanbul The door to the Saudi consulate in Istanbul

For more on this story, watch Panorama: The Khashoggi Murder Tapes on BBC iPlayer and read Jane Corbin's report, The secret tapes of Jamal Khashoggi's murder

Khashoggi’s death quickly became an international scandal.

After an initial, bungled explanation of what might have happened to Khashoggi in Istanbul, the Saudi authorities have gone to great lengths to distance MBS from the scandal.

It was a rogue operation, they said, a case of people grossly exceeding their orders or taking matters into their own hands. But the CIA, and other Western spy agencies, have listened to the gruesome audio tapes that Turkey’s MIT intelligence agency secretly recorded from inside the Saudi Consulate.

And when the US launched sanctions against 17 individuals it suspected of being involved in the murder, Saud al-Qahtani was top of the list.

To date, there is still no “smoking gun” that unequivocally ties MBS to the murder.

But a classified CIA assessment obtained by the Wall Street Journal showed at least 11 text messages sent by MBS to al-Qahtani before, during and after Khashoggi was murdered.

Khashoggi's murder sparked protests across the world Khashoggi's murder sparked protests across the world

And let’s be clear here. In the Gulf Arab states, where I have lived and worked for years, there is no such thing as “a rogue operation”. They simply do not happen. Nothing, but nothing, gets done in the Gulf without sign-off from above.

In August 2018 last year, before the murder, Saud al-Qahtani put out a telling tweet. It ran: “Do you think I make decisions without guidance? I am an employee and an executor of the orders of the King and the Crown Prince.”

No-one has suggested King Salman himself had anything to do with the murder. But the plot does appear to have been hatched from right within his favourite son’s inner circle. “It is inconceivable,” said one former British intelligence officer, “that MBS did not know about it.”

“The Khashoggi murder was a stain on our country, our government and our people” Prince Khalid bin Bandar al-Saud, Saudi ambassador to London

So where is al-Qahtani and why isn’t he on trial?

I put this question recently to the newly appointed Saudi Ambassador to London, Prince Khalid bin Bandar al-Saud. The ambassador assured me that al-Qahtani had been removed from his post and was being investigated. If he is found to be involved he will be prosecuted, he told me. But reports from Riyadh show that although he is keeping a low profile, al-Qahtani has not been detained.

“He is not showing up at court but he’s still involved in cyber-security issues and other projects,” says one Gulf resident with close knowledge of MBS’s circle. “He’s lying low but they are tapping into his expertise.”

The commentator went further, adding that to the people around MBS, al-Qahtani is someone who “took one for the team”. “Yes, the operation [in Istanbul] went wrong but he executed his orders.”

The Saudis have put 11 men on trial over the Khashoggi case. Proceedings began in January and nine months later there is still no word of any convictions or punishments or even an indication of when the trial might conclude.

The Saudi justice system is notoriously opaque at the best of times, with judgements often left to the whim of a judge rather than following any prescribed penal code.

One senior figure who has been named by the authorities is Maj-Gen Ahmed al-Assiri, the deputy head of intelligence and previously the spokesman for the controversial Saudi-led air campaign in the Yemen war.

Maj-Gen Ahmed al-Assiri Maj-Gen Ahmed al-Assiri

Having met him in Riyadh on several occasions, al-Assiri did not strike me as the sort of person to come up with a plan without first getting clearance from above.

Whatever the eventual outcome of the shadowy trial one thing is clear - the Khashoggi murder has done enormous and lasting damage to both MBS’s and Saudi Arabia’s global reputation.

“Let me be clear,” the Saudi ambassador to London told me in September, with surprising frankness for such a senior member of the ruling family. “The Khashoggi murder was a stain on our country, our government and our people. I wish it hadn’t happened.”