Prosecutors began interrogating staff at the Saudi consulate in Istanbul on Friday over the disappearance of journalist Jamal Khashoggi, Turkish news outlets reported.

As many as 15 employees were questioned, including the driver of consul general Mohammed al-Otaiba, who left the country this week, according to the state-run Anatolia News Agency. Other interviewees included technicians, accountants, and secretaries.

Mr Khashoggi, 60, went missing on 2 October after he entered the Saudi consulate to obtain paperwork ahead of his marriage to a Turkish woman.

The well-connected dissident journalist and activist never emerged. Security camera footage and passport counter data released to media suggested 15 Saudi men arrived in Istanbul and headed to the consulate hours before Mr Khashoggi’s scheduled visit, later departing hastily for the airport and other locales in black-tinted vans.

Unnamed officials have reportedly described audio tape evidence of Mr Khashoggi being tortured and killed, and possibly dismembered, but no recordings have emerged.

A man entering Saudi Arabia’s consulate in Istanbul on Friday (AP/Lefteris Pitarakis) (AP)

Tight-lipped officials in Riyadh, under the sway of the country’s de facto ruler crown prince Mohammad bin Salman, have strenuously denied involvement in Mr Khashoggi’s disappearance.

Turkish and Saudi officials this week launched what has been described by officials as a joint investigation into the case. The probe has included searches of the consulate and the residence of the consul general, where some members of the 15-man Saudi team allegedly stopped off for around two hours on the afternoon of 2 October. They were then said to have headed to Istanbul’s Ataturk airport and boarded private jets.

“All the evidence points to it being ordered and carried out by people close to Mohammad bin Salman.” Former MI6 chief Sir John Sawers

The fate of The Washington Post columnist has strained relations between the US and Saudi Arabia, a well-armed, oil-rich behemoth that serves as the US gendarme in the Persian Gulf.

US secretary of state Mike Pompeo swept through Ankara on Thursday, meeting with top officials before heading back to Washington, where he recommended giving the investigation a few more days before taking any action. The administration of president Donald Trump is under tremendous pressure to act, with a group of 22 bipartisan senators triggering the Magnitsky Act, which forces the White House to consider sanctions against foreign leaders accused of human rights violations.

This week, Vermont senator Patrick Leahy, along with seven other Democrat senators, called for an investigation into family and business ties between the Trump administration and Saudi Arabia, upping the pressure on the White House.

“If Mr Khashoggi was tortured and murdered by, or with the knowledge of, the Saudi government, it will be long past time to treat the Saudi royal family as the criminal enterprise that it is,” Mr Leahy said in a statement.

A dozen Indonesian journalists hold posters with photos of Saudi writer Jamal Khashoggi during a protest outside the Saudi embassy in Jakarta, Indonesia, on Friday ((AP Photo/Achmad Ibrahim))

The Turkish government, too, is under pressure to get to the bottom of a case that has drawn international attention as an example of the impunity enjoyed by surging tyrannical regimes across the globe.

Turkey’s main opposition Republican People’s Party, known by the Turkish initials CHP, on Friday called for a parliamentary probe of Mr Khashoggi’s disappearance, arguing the case had harmed the country’s reputation.

“The unserious attitude shown by Saudi consul general in Istanbul Mohammed al-Otaibi and the Saudi government is thought provoking,” said a petition filed by lawmakers.

A former head of MI6, Sir John Sawers, told the BBC on Friday that all the evidence released so far strongly suggests Mr Khashoggi was killed on the orders of the Saudi crown prince, who has spent tens of millions of dollars casting himself as a modernising reformer to fawning western leaders and opinion-makers.