Trump announced he'd dispatched Secretary of State Mike Pompeo to the kingdom - and anywhere else necessary - to get to the bottom of the apparent demise of Khashoggi, a Saudi who had been living and working in the United States.
Saudi Arabia on Sunday warned against threats to punish it over last week's disappearance of journalist Jamal Khashoggi, as European leaders piled on pressure and two more U.S. executives scrapped plans to attend a Saudi investor conference.
More major players in global finance and industry are pulling out of a high-profile business conference in Saudi Arabia in the wake of reports that a Saudi writer critical of the government disappeared and is feared dead after entering a Saudi consulate in Turkey.
"What has reportedly happened in Turkey around the disappearance of journalist Jamal Khashoggi, if proved true, would clearly change the ability of any of us in the West to do business with the Saudi government", billionaire investor Richard Branson said.
Trump said the USA would "get to the bottom of it" and noted there would be "severe punishment" for the Saudis if evidence proved they were involved in the murder of Khashoggi.
"There's something really awful and disgusting about that, if that was the case, so we're going to have to see", Trump said.
The New York Times has taken another strong action against Saudi Arabia in response to the disappearance of Washington Post reporter Jamal Khashoggi.
Future contracts on the S&P 500 index, Dow Jones Industrial Average and Nasdaq were all in the red, indicating that investors remain cautious about US stocks after last week's sharp sell-off.
US Treasury Secretary Steven Mnuchin said he still plans to attend but would "take (it). into account" if more information came out next week. The screening comes after Saudi Arabia lifted a 35-year ban on movies in April when they screened Black Panther.
Erdogan and King Salman spoke by telephone on Sunday evening and stressed the importance of the two countries creating the joint working group as part of the investigation.
"The kingdom affirms its total rejection of any threats and attempts to undermine it, whether by threatening to impose economic sanctions, using political pressures or repeating false accusations", said the statement, carried by the state-run Saudi Press Agency.
In an interview with news broadcaster CBS on Saturday, he warned the USA would be "very upset and angry" if the Saudis ordered Mr Khashoggi's killing.
Saudi Arabia's King Salman on Monday ordered an internal probe into the unexplained disappearance of prominent journalist Jamal Khashoggi as a joint Turkish-Saudi team was set to search the Saudi consulate in Istanbul where he was last seen on October 2.
A range of punishments are under discussion within the US administration, from downgrading diplomatic relations or sanctioning Saudi officials to following major USA companies in withdrawing officials from an investment conference in Riyadh later this month.
This message had been conveyed "directly to the Saudi authorities", said the statement, signed by Britain's foreign minister Jeremy Hunt, his French counterpart Jean-Yves Le Drian and Germany's Heiko Maas.
The drop in the Tadawul exchange in Riyadh happened Sunday, the first day of trading.
Trump told CBS' "60 Minutes" on Sunday that the consequences of Saudi involvement in foul play involving the journalist would be "severe".
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