Khashoggi, a Saudi national and United States resident who became increasingly critical of Prince Mohammed in columns for the Washington Post, has not been seen since he walked into the kingdom's Istanbul consulate to sort out marriage paperwork on Oct 2.
Tensions between Washington and Riyadh over the disappearance of Mr Khashoggi on October 2 after his visit to the Saudi consulate in Istanbul increased over the past week.
"Been hearing the ridiculous "rogue killers" theory was where the Saudis would go with this", Sen.
Khashoggi has written extensively for the Washington Post about Saudi Arabia, criticising its war in Yemen, the recent diplomatic spat with Canada and its arrest of women's rights activists after the lifting of a driving ban for women.
CNN reported that the Saudis were going to admit the killing happened but deny the king or crown prince had ordered it - which does not match what analysts and experts know about the kingdom's inner workings.
One of those two individuals said he was told by those close to the Saudi leadership that the kingdom will claim that rogue operatives killed Khashoggi during an interrogation or a rendition attempt that went horribly awry.
"It has been 13 days since the event, so surely proving some of the evidence might be hard, but we believe we will obtain evidence", the Turkish official said.
In this April 28, 2018 photo released by Saudi Press Agency, SPA, and made available April 29, 2018, US Secretary of State Mike Pompeo, center left, is received by Saudi Foreign Minister Adel Al Jubeir, in Riyadh, Saudi Arabia.
Investigators entered the building in Istanbul on Monday afternoon - first a Saudi team followed roughly an hour later by Turkish forensic police.
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King Salman and Turkish President Tayyip Erdogan spoke by telephone on Sunday evening and stressed the importance of the two countries creating a joint group as part of the probe.
The Saudi authorities "have at their disposal tens of options [to retaliate]", said Abdulkhaleq Abdulla, a political science professor at the UAE University.
Saudi Arabia is reportedly ready to concede that a missing Saudi writer was killed in its consulate in Istanbul.
King Salman emphasised the importance of the Turkey-Saudi relationship and said no one should be able to "undermine the strength of this relationship", Saudi's official media reported.
Speaking of the Holocaust, Khashoggi called the chapter a frightful episode in history, but also said that there were other awful events throughout history, such as the burning of Baghdad by the Mongols in the Middle Ages or the Palestinian Nakba. European allies have urged "a credible investigation" and accountability for those responsible.
Khashoggi, a familiar face on Arab talk shows, moved to the United States previous year fearing retribution for his criticism of Prince Mohammed, who has cracked down on dissent with arrests.
The Saudi riyal fell to its lowest in two years and its worldwide bond prices slipped over fears that foreign investment could shrink amid global pressure.
Trump had dispatched Pompeo to speak to the monarch of the world's top oil exporter over Khashoggi's disappearance. But Khashoggi's disappearance has led several business leaders and media outlets to back out of the upcoming investment conference in Riyadh, called the Future Investment Initiative.
The Uber investment was the PIF's first major deal in the technology start-up market, signaling its shift in strategy toward more aggressive and eye-catching foreign investments.