Jamal Khashoggi had weighed the risks before he walked into the Saudi consulate in Istanbul last Tuesday. He had sought assurances from friends, given a number for his fiancee to call if he failed to reappear – and even received an overture to return to Riyadh from Mohammed bin Salman himself.

Convinced that he faced no official threat from the powerful crown prince, and feeling pressure from his future father-in-law to get the marriage process rolling, Khashoggi believed the odds were with him.

Five days later, the move by the kingdom’s highest-profile dissenter to start a new life may have instead led to his death. Khashoggi has not been heard from since. And with each passing day, Turkish authorities are more convinced that he will never be seen again, his life ended by Saudi agents who had lain in wait, then carried out an act of state-sanctioned murder in the heart of Istanbul.

By Sunday night, authorities in Turkey had pieced together a case against Saudi agents that has reverberated through Ankara and Riyadh, placing two regional juggernauts on a diplomatic collision course and underscoring – yet again – the high price of dissent in an era of fast-ebbing freedoms.

The allegations against Riyadh are breathtaking. Khashoggi, say Turkish officials, was tortured, then killed and his body smuggled out of the consulate, all under the nose of local agencies. Initially stunned into silence, or curbed by their political masters, officials had initially remained mute as questions about Khashoggi’s disappearance mounted. But at midnight on Saturday, that changed, when a government press officer circulated a news report quoting two Turkish sources who had in turn claimed that Khashoggi was likely dead.

Ever since, officials from across the country’s executive and security apparatus have drip-fed information from a still-ongoing investigation. Sources inside the consulate have helped shape understandings, as have hundreds of hours of security and flight logs detailing the comings and goings of a 15-person Saudi security delegation and – according to an ally of Khashoggi – the route taken by a convoy that left the consulate on Tuesday, hours after he had walked through the gate.

“They followed the cars, and they know what happened,” said Turan Kışlakçı, the head of the Turkish Arab Media Association, and a friend of Khashoggi. “We have all the details, and he was killed.”

According to Kışlakçı, Khashoggi had confided in him in the lead-up to his ill-fated visit. “He said, ‘I’m not afraid, because there is no official investigation against me. On the contrary, recently, Mohammed bin Salman asked me to be his adviser, and I refused, saying this is against my country and region’s interests.’

“He said, ‘the most they can do is interrogate me. And I can give them answers, I have nothing to hide.’” His fiancee’s father pressurised him to get the relevant documents to initiate the official marriage process in Turkey. His trusted Saudi friends in the US gave him assurance. He was confident in what he was doing.”

Jamal Khashoggi in 2011 Photograph: Virginia Mayo/AP

Riyadh has vehemently denied playing a role in Khashoggi’s disappearance. Prince Mohammed said in an interview last Friday that “if he was [in Riyadh] I would know”. Seemingly unwilling – at this point – to publicly call their regional rival out, Turkey has so far taken a cautious stance. Turkey’s president, Recep Tayyip Erdoğan, said on Sunday he was “saddened” by Khashoggi’s disappearance and intended to wait for the results of an investigation.

The Turkish leader’s position reflects the delicate balance of interests between Ankara and Riyadh, two regional heavyweights who remain at odds on pivotal issues, such as the Saud-led blockade with Qatar, the status of the Muslim Brotherhood and the 2013 military coup in Egypt. Turkey remains resolutely in Doha’s camp on the Qatar standoff, however its officials prefer to direct their enmity at the United Arab Emirates, rather than Riyadh, with which they try to remain on neutral terms.

By allowing state functionaries – not elected officials – to out the Saudis over Khashoggi’s vanishing, Turkey has dealt itself a powerful hand, one which Erdoğan fully intends to play in the coming week, as intelligence officials prepare to table their findings.

The concessions the leader of a debt-burdened economy could potentially squeeze from a Saudi leader facing allegations that the kingdom’s agents had murdered a foe in a foreign city could be significant. But that would depend on how much the Saudi heir to the throne fears the scrutiny.

Less than one year since Prince Mohammed effectively kidnapped the Lebanese prime minister, Saad Hariri, after summoning him to Riyadh, Hariri was forced to sign over assets worth hundreds of millions of dollars, as were many of the kingdom’s business and political elite.

Since then, Prince Mohammed has pushed ahead with social and economic reforms, but intensified a crackdown on political freedoms. The brash 33-year-old has shown no tolerance for dissent. In the kingdom’s eyes, Khashoggi – through his platform in the Washington Post and access to stakeholders in the US and Europe – was more than a critic: he had become a threat.

Saudi denials – and deflection – of any knowledge of Khashoggi’s whereabouts read from a Kremlin playbook, as does his alleged death.

Turkey, meanwhile – the world’s most prolific jailer of journalists – now finds itself in the peculiar position of being a champion for a columnist who vanished under its nose.