Saudi Arabia's crown prince has "totally denied" any knowledge behind the disappearance of journalist Jamal Khashoggi to Donald Trump.
The US president tweeted on Tuesday that he and Mohammed bin Salman had discussed the matter directly.
It comes after Secretary of State Mike Pompeo flew to Riyadh in a bid to put pressure on the Saudi regime.
Fears are growing that Mr Khashoggi, who disappeared after visiting the Saudi consulate in Istanbul two weeks ago, has been murdered.
President Trump said that during a conference call with bin Salman the crown prince promised a full investigation, adding that answers would be "forthcoming shortly".
"Just spoke with the Crown Prince of Saudi Arabia who totally denied any knowledge of what took place in their Turkish Consulate," the US leader tweeted.
"He was with Secretary of State Mike Pompeo during the call, and told me that he has already started, and will rapidly expand, a full and complete investigation into this matter. Answers will be forthcoming shortly."
On Monday, President Trump spoke with the Gulf nation's King Salman, who also denied knowing who was behind the disappearance.
The US leader told reporters that King Salman had implied to him that "rogue killers" could have been involved.
"We are going to try to get to the bottom of this very soon. But, his was a flat denial," Mr Trump said.
Pressure is continuing to grow on Saudi authorities to reveal what exactly happened to Mr Khashoggi.
A writer with for the Washington Post, he was a vocal critic of the Saudi regime.
Mr Khashoggi was last seen when he visited the consulate in Turkey to pick up paperwork he needed to get married.
On Tuesday, unnamed Turkish officials claimed that evidence had been found in the diplomatic building to suggest Mr Khashoggi was killed there.
Saudi officials have called Turkish allegations that a team of 15 Saudi agents killed Khashoggi "baseless," but US media reports suggested that the kingdom may acknowledge the writer was killed at the consulate, perhaps as part of a botched interrogation.
They have also also rejected any "threats" of economic sanctions or political pressure from the US over the disappearance.