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Saudi Arabia's powerful Crown Prince Mohammed Bin Salman has denied knowing what happened to the missing Saudi journalist, Jamal Khashoggi, according to President Donald Trump.

Mr Trump tweeted that Prince Mohammed had spoken to him on the phone.

Unnamed Turkish officials say Monday's search of the Istanbul Saudi consulate has turned up more evidence Mr Khashoggi was killed there.

The issue has put Saudi Arabia under pressure from close allies.

The journalist, a critic of the Saudi leadership, was last seen alive entering the consulate on 2 October. Saudi Arabia has denied killing him and initially said he left the building unharmed.

What has Prince Mohammed said?

The Saudi heir apparent wields considerable power in the kingdom and is being held responsible by many outside for whatever happened to Mr Khashoggi.

Mr Trump tweeted that he "totally denied any knowledge of what took place in their Turkish Consulate".

The president said he "told me that he has already started, and will rapidly expand, a full and complete investigation into this matter. Answers will be forthcoming shortly".

The phone call coincided with a visit to Saudi Arabia by US Secretary of State Mike Pompeo, sent there by Mr Trump to deal with the crisis in relations.

On Monday, Mr Trump speculated that "rogue killers" may have been behind the journalist's disappearance.

In a further sign of disquiet within the US, a leading Republican senator and defender of US-Saudi links, Lindsey Graham, has been highly critical of the Saudi crown prince.

"This guy's gotta go," he said on Tuesday morning on Fox News, describing Prince Mohammed as a "wrecking ball".

Reputations may be forever tainted

By Frank Gardner, BBC News

The recent, highly charged exchange of words between Washington and Riyadh now appears to have given way to a mutual search for the least bad explanation. Both countries' leaders know they have an enormous amount to lose if this affair ends up splitting apart their 73-year-old strategic partnership.

Iran, as the regional rival to Saudi Arabia, would be the prime beneficiary if the Saudis were to lose their defensive US umbrella. President Trump is also correct when he says thousands of US jobs would be lost, with China and Russia to be among those lining up to replace them.

Which begs the wider question: is the West's relationship with Saudi Arabia so important that it outweighs the need to condemn and punish what many believe was a state-sponsored murder of a journalist inside a consulate?

Hence the urgent dispatching of the US secretary of state for talks with the Saudi leadership. In private there may well be some strong words, in public both countries may want to present a united stand. But one thing is certain: whatever narrative emerges, the international reputation of the Saudi crown prince and power-behind-the-throne Mohammed Bin Salman will forever be tainted by this affair.

What is the state of investigation?

The focus has expanded from the Istanbul consulate to a search of the consul's residence, some 200 metres away.

On the day of Mr Khashoggi's disappearance, several vehicles with Saudi diplomatic number plates were seen on video footage moving from the consulate to the residence.

The consul, Mohammad al-Otaibi, left Turkey on a commercial flight bound for Saudi Arabia, hours before Turkish police were due to enter the building on Tuesday.

Meanwhile, Turkish investigators quoted by two news agencies say Monday's search of the consulate has yielded more evidence that Mr Khashoggi was killed there.

A Turkish official is quoted by Reuters as saying "strong evidence" was found but no conclusive proof.

Turkish security sources have long made clear they have audio evidence pointing to his murder.

The search of the consulate continued into the early hours of Tuesday. Samples, including soil from the garden and a metal gate, were reportedly taken.

Turkish President Recep Tayyip Erdogan told reporters the investigation "was looking into many things such as toxic materials and those materials being removed by painting them over".

The New York Times and CNN, quoting unnamed sources, reported on Monday that Saudi Arabia would acknowledge that Mr Khashoggi's death was the result of an interrogation that went wrong.

What is alleged to have happened at the consulate?

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Mr Khashoggi, who has written for the Washington Post and is a US resident, arrived at the consulate at 13:14 local time on 2 October to obtain paperwork so he could marry his Turkish fiancée, Hatice Cengiz.

Reports suggest an assault and struggle took place in the building.

Turkish sources allege he was killed by a 15-strong team of Saudi agents pictured entering and leaving Turkey on CCTV footage released to media outlets.

There are allegations in Turkish media that his body was dismembered in the building.

Mr Khashoggi was once an adviser to the Saudi royal family but fell out of favour with the country's rulers and went into self-imposed exile.