According to Turkish officials, initial investigations indicated the Saudi national was murdered there.
Jamal Khashoggi, 59, went missing after visiting the Saudi consulate in Istanbul on October 2.
Saudi Arabia has denied the accusations, saying it is “working to search for him”.
The Washington Post said it would be a “monstrous and unfathomable act” if Jamal Khashoggi had been killed.
Turkish officials have opened an investigation and have been speaking to the media on condition of anonymity.
They have not given any evidence for their claim, nor suggest how he was killed.
Jamal Khashoggi is a high-profile critic of Saudi Arabia’s Crown Prince Mohammed bin Salman. The journalist has more than 1.6 million Twitter followers and has written for the Washington Post opinion section.
On October 2, he went to the consulate to obtain a document certifying he had divorced his ex-wife, so that he could marry his Turkish fiancée, Hatice Cengiz.
Hatice Cengiz said she waited outside for 11 hours, but he did not come out.
She said Jamal Khashoggi was required to surrender his mobile phone, which is standard practice in some diplomatic missions. He told her to call an adviser to Turkish President Recep Tayyip Erdogan if he did not return.
The head of the Turkish-Arab Media Association, Turan Kislakci, told the New York Times that Turkish police officers providing security for the consulate had checked their security cameras and did not see the journalist leave on foot. However, diplomatic cars had been seen moving in and out.
On October 3, Prince Mohammed bin Salman told Bloomberg News that Turkish authorities were welcome to search the building because “we have nothing to hide”.
He said: “He’s a Saudi citizen and we are very keen to know what happened to him. And we will continue our dialogue with the Turkish government to see what happened to Jamal there.
“My understanding is he entered and he got out after a few minutes or one hour. I’m not sure. We are investigating this through the foreign ministry to see exactly what happened at that time.”
When asked if Jamal Khashoggi faced charges in Saudi Arabia, the crown prince said his country would need to know where he was first.
Jamal Khashoggi is one of the most prominent critics of the crown prince, who has unveiled reforms praised by the West while carrying out an apparent crackdown on dissent. Human and women’s rights activists, intellectuals and clerics have been arrested – meanwhile, Saudi Arabia is waging a war in Yemen that has triggered a humanitarian crisis.
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