In early 2016, when Mohammed bin Salman was still deputy crown prince and Donald Trump still a contender for US president, the then 30-year-old Saudi summoned senior British officials to Riyadh to see him. He had one thing on his mind, said two of the officials present that day – how to deal with Vladimir Putin.

The Russian president’s role in the Middle East had suddenly expanded and his footprint throughout Europe and the US was growing just as rapidly. The young prince seemed curious about what the mercurial Putin had been up to: annexation, intimidation, deflection, the denial of objective facts. But he kept coming back to one question, the officials recalled: how does he get away with it? “He was fascinated by him,” one of the Britons told the Observer. “He seemed to admire him. He liked what he did.” Two years later Prince Mohammed is embroiled in a crisis unlike any other in his short, combustible time as the world’s most powerful thirtysomething. The crown prince stands accused of ordering the brutal death of a prominent critic on foreign soil – a state-sanctioned hit that is without precedent in the kingdom’s modern history, but is not quite so unknown in Russia.

Play Video 1:00 Jamal Khashoggi: CCTV shows alleged Saudi hit squad's movements – video

The events, as described by Turkish officials, are staggering and have shaken confidence in Prince Mohammed even among his closest allies, who until the past week had been steadfast in their support for his ambitious reform programme. Turkish intelligence and senior officials insist that the Saudi journalist Jamal Khashoggi was killed inside the Saudi consulate in Istanbul by a 15-man hit squad that had arrived from Riyadh on the same day and then dismembered his body.

Ever since, authorities have sanctioned a drip-feed of leaks: of video footage showing Khashoggi entering the building on the afternoon of 2 October, the names of the 15 Saudis who arrived – all of whom were linked to the state security apparatus – and the flight logs of the private jets they flew in and out on. The jets were rented from a Riyadh company that routinely leases planes to the Saudi government.

Trump vows 'severe punishment' if Saudis killed Jamal Khashoggi Read more

Turkish intelligence officers have told their counterparts in the CIA that they have an audio recording and partial videotape of the moment Khashoggi was killed. They have suggested that the end of Khashoggi’s life was captured on an Apple watch he was wearing that was synced to an iPhone held by his fiancee, Hatice Cengiz, waiting outside. Suspicion in western intelligence circles, however, is that the Turks had the consulate bugged. Khashoggi has not been seen since he entered the building, and Riyadh has been unable to produce evidence supporting its claim that he walked away a free man after finalising his divorce papers. Faced with evidence, Saudi officials have offered strenuous denials and deflection; this, they say, was a conspiracy led by regional foe Qatar, supported by its allies in Ankara.

The state television network Al Arabiya has even raised the Salisbury spire defence, claiming the 15 Saudis, who came and went within hours, were “tourists” – just like the Russian assassins widely accused of poisoning the Russian double agent Sergei Skripal and his daughter Yulia with a state-made nerve agent in the Wiltshire cathedral city.

Now nearing a third year in office, Trump, who has embraced the man known throughout Washington as MbS, has been forced into uncharacteristically cautious language when addressing the allegations, even if he warned there would be “severe punishment” for Riyadh, if it turned out that Khashoggi had indeed been killed in the consulate.

Having a robust regional voice as a bulwark against Iran has been the central plank of Trump’s foreign policy. And so far the US leader seems to view the allegations as an inconvenience, not a game changer.

Last week the US national security adviser, John Bolton, appeared to add weight to Riyadh’s conspiracy claims, suggesting that Ankara and Riyadh had long been rivals, and that some kind of “operation” might have taken place.

The Trump family’s ties to MbS extend beyond a shared view on Iran. The president’s son-in-law, Jared Kushner, has established warm personal and professional ties, as have key members of an influential US business network and the secretary of state, Mike Pompeo, who has spent dozens of hours with the crown prince in his royal diwan, or a tent in the desert, where he often retreats for weekends.

European officials in Riyadh and elsewhere in the region say MbS has drawn inspiration from the fact that Trump, unlike most other modern US leaders, has disavowed making humanitarian concerns a pillar of his foreign policy – and has openly embraced authoritarianism.

Trump repeatedly claims that the US no longer sets the world order – except on Iran – and has pointedly eased pressure on states that previous administrations claimed had long fallen short on human rights and governance. The result has been a vacuum in global leadership, emboldening leaders who might otherwise have checked their behaviour, a Riyadh-based western ambassador said.

“Relationships are being redefined. People know there are no limits,” the ambassador said. “If ever there was a case study of the post-fact Middle East, this is it. There is impunity all around.”

Inside the kingdom, where a crackdown on dissent has left much of society unwilling to speak publicly, there is widespread belief that Khashoggi’s disappearance was indeed a conspiracy orchestrated by its enemies. Some former security officials, however, sense that it may instead mark a crack in the new regime.

“The realignment of the [national intelligence service reporting directly to MbS] has been a disaster,” said an adviser to one such official. “It has become a hyper-political tool for the diwan … that has removed any semblance of accountability and encouraged abuse of power on a scale I have not witnessed before in modern times. There has been a broad descent into authoritarianism. The scale and pace of repression have destroyed any embryonic seeds of civil society and discourse. There really is no limit to the consequences of even polite disagreement, let alone dissent.”

With much of Turkey’s evidence against Saudi Arabia now publicly laid bare, Ankara – and Riyadh – are both turning to Washington to find a way out of a crisis with seemingly endless dimensions. Turkey on Friday agreed to a joint probe with Riyadh into what took place – an arrangement that was brokered by senior officials, suggesting power politics may end up taking precedence over the truth behind Khashoggi’s disappearance.

Turkey does not need investigative help to establish Khashoggi’s fate. Listening devices and camera footage, which it has not yet revealed, offer incriminating evidence. It does, however, need political cover to navigate a problem that could have significant trade and investment considerations.

Riyadh faces a more imminent blow to its trade agenda, with an investment conference set for 23 October at risk of a boycott from media partners and high-profile global companies, in protest at the lack of answers from Saudi officials.

“They have belatedly realised the stakes in this,” said one of the British officials at the 2016 meeting with the crown prince. “And he has been shocked to learn that the absolute power he has at home he doesn’t have abroad. Even in Turkey there are rules. Putin learned the same in the UK. And if this guy is going to survive this, he is going to be indebted to Ankara and Turkey for sparing everyone from the shocking truth.”