Former U.S. Ambassador to Saudi Arabia Robert Jordan on Monday warned President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE against accepting the Saudi government's denials about the disappearance of a Washington Post columnist.

“Let’s remember, this is the same King Salman who told me after 9/11 that the 9/11 attacks were an Israeli plot,” Jordan said. “He said that firmly. Did I believe that? Of course not. I don’t think you can go in with wide-eyed acceptance of anything some of these world leaders say.”

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Jordan added that it's possible Salman did not have any knowledge of journalist Jamal Khashoggi's disappearance, and that the situation was in the hands of the crown prince, Mohammad Bin Salman.

"But nonetheless, I think we’ve got to have a thorough investigation," he said.

Khashoggi, a U.S. resident and outspoken critic of the Saudi government, went missing after entering the Saudi consulate in Istanbul. The Turkish government has said it has video and audio recordings that prove Khashoggi was killed.

But Saudi Arabia is denying involvement.

"Just spoke to the King of Saudi Arabia who denies any knowledge of whatever may have happened 'to our Saudi Arabian citizen,'" Trump tweeted on Monday.

"He said that they are working closely with Turkey to find answer. I am immediately sending our Secretary of State to meet with King!"

Trump later said that Mike Pompeo Michael (Mike) Richard PompeoTreasury sanctions individuals, groups tied to Russian malign influence activities Navalny released from hospital after suspected poisoning Overnight Defense: Pentagon redirects pandemic funding to defense contractors | US planning for full Afghanistan withdrawal by May | Anti-Trump GOP group puts ads in military papers MORE would depart for Saudi Arabia and that his trip may include a visit to Turkey on Monday.

Trump has said that Saudi Arabia would face "severe punishment" if it was responsible for Khashoggi's disappearance. But he has been cautious about altering arms deals between the two nations, saying there are “other ways of punishing” Saudi Arabia.

Saudi Arabia permitted investigators to search the consulate in Istanbul, according to CNN.