US President Donald Trump has suggested "rogue killers" could be behind the disappearance of Saudi journalist Jamal Khashoggi in Turkey.
Turkish officials believe Mr Khashoggi was murdered in the consulate by Saudi agents nearly two weeks ago but Riyadh has always strongly denied this.
The issue has strained Saudi Arabia's ties with its closest Western allies.
Speaking to reporters after a phone call with King Salman, Mr Trump said the Saudi leader had firmly denied knowing what had happened to Mr Khashoggi.
Mr Trump addressed snatched questions from reporters over helicopter engine noise at the White House, describing King Salman's denial as "very, very strong".
"It sounded to me like maybe these could have been rogue killers," he added. "Who knows?"
The president provided no evidence to back his comment.
CNN reported Saudi Arabia was preparing a report that would say Mr Khashoggi was killed as the result of an interrogation that went wrong,citing two unidentified sources. One cautioned that the report was still being prepared and could change, CNN said. The Saudi government could not immediately be reached for comment.
Us Secretary of State Mike Pompeo is flying to Saudi Arabia immediately.
Saudi and Turkish investigators have been seen entering the Saudi consulate where Mr Khashoggi was last seen.
A Turkish security source has told the BBC that officials have audio and video evidence proving Mr Khashoggi was murdered inside the building.
On Saturday, Mr Trump threatened Saudi Arabia with "severe punishment" if it emerged that Mr Khashoggi had been killed inside the consulate but ruled out halting big military contracts with Riyadh.
Diplomatic pressure is growing on the Saudis to give a fuller explanation.
On Monday, King Salman ordered an investigation into the case.
"The king has ordered the public prosecutor to open an internal investigation into the Khashoggi matter based on the information from the joint team in Istanbul," an official quoted Reuters.
The official said the prosecutor had been instructed to work quickly.
Last week, Turkey accepted a Saudi proposal to form a joint working group to investigate Mr Khashoggi's disappearance.
On Sunday, Riyadh angrily rejected political and economic "threats" over the case of the missing journalist and said it would respond to any punitive action "with a bigger one".
Investigators have been arriving at the building in Istanbul - first a Saudi team followed by Turkish forensic police.
Turkish diplomatic sources had said the consulate would be searched by a joint Turkish-Saudi team.
A group of cleaners was seen entering earlier.
Saudi Arabia agreed last week to allow Turkish officials to conduct a search but insisted it would only be a superficial "visual" inspection.
Turkey rejected that offer. The Sabah daily newspaper said investigators had wanted to search the building with luminol, a chemical which shows up any traces of blood.
Mr Khashoggi, a critic of the Saudi government who has written for the Washington Post, was last seen walking into the consulate on 2 October.
Reports suggest an assault and struggle took place in the consulate after Mr Khashoggi went to get paperwork for his forthcoming marriage.
Turkish sources allege he was killed by a 15-strong team of Saudi agents but Riyadh insists that he left the consulate unharmed.
Mr Khashoggi was once an adviser to the Saudi royal family but fell out of favour with the Saudi government and went into self-imposed exile. He is a US resident.
Meanwhile, more leading business figures say they will not attend a major investment conference in Riyadh later this month.
The head of JP Morgan, Jamie Dimon, is one of the latest high-profile executives to pull out. Ford chairman Bill Ford and Uber chief executive Dara Khosrowshahi are also among those who will not be attending the conference.
It is still unclear whether US Treasury Secretary Steve Mnuchin will attend the conference.
- BBC / Reuters