Istanbul (CNN) President Donald Trump suggested Monday that "rogue killers" could be behind the disappearance of Saudi journalist Jamal Khashoggi, after a phone call with Saudi Arabia's King Salman about the case.

Trump told White House reporters that Salman offered him a "flat denial" in relation to the disappearance of Khashoggi, a columnist for The Washington Post who was last seen in public when he entered the Saudi consulate in Istanbul in Turkey on October 2.

Later Monday, the Saudis were preparing to admit that Khashoggi died during an interrogation that went wrong, one that was intended to lead to his abduction from Turkey, according to two sources.

Secretary of State Mike Pompeo left Washington for Riyadh at around midday to meet with the Saudi King on Trump's orders.

Previously, Saudi authorities had maintained that Khashoggi left the consulate the same afternoon of his visit, but have provided no evidence to support the claim. Khashoggi's fiancée, Hatice Cengiz, who was waiting outside the consulate, says she did not see him re-emerge.

A security officer peers through a partially open door at the Saudi consulate in Istanbul on Friday.

"It wasn't like there was a question in his mind. The denial was very strong," Trump said of King Salman.

"With that being said," Trump continued, "the king firmly denied any knowledge of it. He didn't really know -- maybe -- I don't want to get into his mind but it sounded to me like maybe these could have been rogue killers, who knows," Trump said.

"We are going to try to get to the bottom of it very soon," Trump added. "But his was a flat denial."

CNN reporters saw Turkish investigators, including forensics officers, entering the Saudi consulate in Istanbul Monday evening. Saudi officials had granted permission for the premises to be searched, a Turkish diplomatic source told CNN, and police were seen cordoning off the area before investigators arrived. Turkish officials also want to search the nearby consul general's residence.

Turkish authorities believe 15 Saudi men who arrived in Istanbul on October 2 were connected to Khashoggi's disappearance and possible murder. At least some of them appear to have high-level connections in the Saudi government.

A source familiar with the ongoing investigation told CNN on Friday that Turkish authorities have audio and visual evidence that shows journalist Khashoggi was killed inside the consulate.

International pressure is mounting on Riyadh to explain the journalist's disappearance, in a case that has created a diplomatic rift between Saudi and the West. The UK, France and Germany have demanded a "credible investigation" into the events and Trump on Sunday warned of serious retribution if the Saudis were found to be behind his possible death.

Saudi officials threatened to retaliate if the US imposed sanctions, but Riyadh later softened its tone.

The case has also caused friction between Saudi Arabia and Turkey, which has repeatedly accused the Saudis of failing to cooperate with their investigation.

CCTV footage shows Khashoggi entering the Saudi consulate on October 2.

The agreement to allow a search of the consulate comes a day after King Salman called Turkish President Recep Tayyip Erdogan to discuss the case. The two leaders agreed on a joint working group to look into the journalist's disappearance.

The Saudi official said that a royal decree was issued Friday directing the public prosecutor to conduct an internal investigation into the Khashoggi case, based on intelligence shared by Turkey with Saudi Arabia.

The official said that although a joint investigation team was cooperating on the ground, the Saudi "leadership had felt that an internal investigation was needed to make sure no stone is left unturned to unfold the truth."

International companies pull out

Amid the diplomatic fallout over Khashoggi's disappearance, international firms are pulling out of a high-profile investment summit. The Future Investment Initiative conference, dubbed "Davos in the Desert," was due to take place later this month in Riyadh, hosted by Saudi Arabia's de facto ruler, Crown Prince Mohammed bin Salman.

The latest high-profile invitee to pull out of the conference is World Bank President Jim Yong Kim, according to a person familiar with the decision. JP Morgan Chase CEO Jamie Dimon has also withdrawn, a spokesperson confirmed Sunday.

US Treasury Secretary Steven Mnuchin was still due to attend the conference. Trump said on Monday that Mnuchin has until Friday to decide whether to attend. "He's going to make that decision," Trump said.

The BBC reported that the attendance of British Trade Secretary Liam Fox was in doubt.

The Kingdom of Saudi Arabia considered delaying the Saudi investment conference until after the investigation, according to a source with direct knowledge of the situation. It is unclear if that is still under consideration, according to the source. CNN received no response when reaching out to the CEO, managing director, and organizers of the conference.

Saudi Arabia has found support in a number of Arab allies, including Oman, Bahrain, Jordan, Lebanon, the United Arab Emirates and Egypt, which all put out statements Sunday saying they expressed solidarity with Saudi Arabia. The Palestinian Authority also put out a statement of support.