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US President Donald Trump has suggested “rogue killers” could be behind the disappearance of Saudi journalist Jamal Khashoggi in Turkey.
Speaking to reporters after a phone call with King Salman, he said the Saudi leader had firmly denied knowing what had happened to Mr Khashoggi.
Secretary of State Mike Pompeo is flying to Saudi Arabia immediately.
Saudi and Turkish investigators are due to search the Saudi consulate where Mr Khashoggi was last seen.
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They will search the building in Istanbul later on Monday, the Turkish foreign ministry says.
Turkish officials believe Mr Khashoggi was murdered in the consulate by Saudi agents nearly two weeks ago but Riyadh strongly denies this.
Mr Trump described King Salman’s denial as “very, very strong”.
“It sounded to me like maybe these could have been rogue killers,” he added. “Who knows?”
How are the Saudis reacting?
Diplomatic pressure is growing on the Saudis to give a fuller explanation.
On Monday, Saudi Arabia’s King Salman ordered an investigation into the case.
“The king has ordered the public prosecutor to open an internal investigation into the Khashoggi matter based on the information from the joint team in Istanbul,” an official quoted by Reuters news agency said.
The official said the prosecutor had been instructed to work quickly.
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Last week, Turkey accepted a Saudi proposal to form a joint working group to investigate Mr Khashoggi’s disappearance.
The latest moves come as more leading business figures say they will not attend a major investment conference in Riyadh later this month.
The head of JP Morgan, Jamie Dimon, is one of the latest high-profile executives to pull out.
Dubbed “Davos in the Desert”, the conference, starting on 23 October, was expected to showcase Crown Prince Mohammed bin Salman’s ideas for reform.
The 33-year-old is widely seen as the most influential figure in the kingdom.
When will the search take place?
While the foreign ministry has confirmed to the BBC that a search will be carried out, details have not been revealed.
Turkish diplomatic sources said the consulate would be searched by a joint Turkish-Saudi team in the late afternoon or evening.
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Police barricades have been set up in front of the Saudi consulate in Istanbul
Saudi Arabia agreed last week to allow Turkish officials to search the building but insisted it would only be a superficial “visual” inspection.
Turkey rejected that offer. The Sabah daily newspaper said investigators had wanted to search the building with luminol, a chemical which shows up any traces of blood.
King Salman and Turkish President Recep Tayyip Erdogan spoke by telephone on Sunday evening, officials said, and stressed the importance of the two countries working together on the case.
What allegedly happened in Istanbul?
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Mr Khashoggi, a critic of the Saudi government who has written for the Washington Post, was last seen walking into the consulate on 2 October.
A Turkish security source has told the BBC that officials have audio and video evidence proving Mr Khashoggi was murdered inside the building.
Reports suggest an assault and struggle took place in the consulate after Mr Khashoggi went to get paperwork for his forthcoming marriage.
Turkish sources allege he was killed by a 15-strong team of Saudi agents but Riyadh insists that he left the consulate unharmed.
Mr Khashoggi was once an adviser to the Saudi royal family but fell out of favour with the Saudi government and went into self-imposed exile. He is a US resident.
How have other countries reacted?
On Saturday, Mr Trump threatened Saudi Arabia with “severe punishment” if it emerged that Mr Khashoggi had been killed inside the consulate.
In an interview with CBS News, Mr Trump said that, if true, the fact that a journalist had been murdered was “terrible and disgusting”.
However, he ruled out halting big military contracts with Riyadh.
On Sunday, Riyadh angrily rejected political and economic “threats” over the missing journalist and said it would respond to any punitive action “with a bigger one”.
The UK, Germany and France have called for a “credible” investigation into the disappearance.
Their foreign ministers said that if anyone were found responsible they should be held accountable, and urged a detailed response from Riyadh.
UK Foreign Secretary Jeremy Hunt said that whatever happened now was “absolutely up to Saudi Arabia”.