The CIA believes that Saudi Crown Prince Mohammed bin Salman ordered the murder of journalist Jamal Khashoggi, according to US media reports.
Sources close to the agency said it had assessed the evidence in detail.
It is understood there is no “smoking gun” but US officials think such an operation would need the prince’s approval.
Saudi Arabia has called the claim false and insisted that the crown prince knew nothing of plans for the killing.
Meanwhile, US Vice-President Mike Pence vowed on Saturday to hold Khashoggi’s killers to account.
Speaking on the sidelines of a summit in Papua New Guinea, he said the US was “determined to hold all of those accountable who are responsible for that murder”.
The journalist was killed in the Saudi consulate in Istanbul on 2 October. His body has not been found.
Turkey insists the order came from the highest levels.
The Washington Post, which Khashoggi worked for, says the CIA assessment was based partly on a phone call made by the crown prince’s brother, Prince Khaled bin Salman, the Saudi ambassador to the US.
Prince Khaled allegedly called Khashoggi at the direction of his brother and gave him assurances that he would be safe to go to the consulate.