Saudi Arabia's powerful Crown Prince Mohammed Bin Salman has denied knowing what happened to the missing Saudi journalist, Jamal Khashoggi, according to President Donald Trump.
Saudi Arabia being blamed was a case of "guilty until proven innocent", Mr Trump told the AP news agency.
Unnamed Turkish officials say a search of the Istanbul Saudi consulate gave more evidence Mr Khashoggi was killed.
The issue has put Saudi Arabia under pressure from close allies.
The journalist, a critic of the Saudi leadership, was last seen alive entering the consulate on 2 October. Saudi Arabia has denied killing him and initially said he left the building unharmed.
What has Prince Mohammed said?
The Saudi heir apparent wields considerable power in the kingdom and is being held responsible by many outside for whatever happened to Mr Khashoggi.
Mr Trump tweeted that Prince Mohammed had spoken to him on the phone and he "totally denied any knowledge of what took place in their Turkish Consulate".
The president said he "told me that he has already started, and will rapidly expand, a full and complete investigation into this matter. Answers will be forthcoming shortly".
The phone call coincided with a visit to Saudi Arabia by US Secretary of State Mike Pompeo, sent there by Mr Trump to deal with the crisis in relations.
On Monday, Mr Trump speculated that "rogue killers" may have been behind the journalist's disappearance.
In a further sign of disquiet within the US, a leading Republican senator and defender of US-Saudi links, Lindsey Graham, has been highly critical of the Saudi crown prince.
"This guy's gotta go," he said on Tuesday morning on Fox News, describing Prince Mohammed as a "wrecking ball".