This incident, which involves the Saudi kingdom, Turkey and the United States has sparked an worldwide uproar.
Khashoggi, a USA resident and critic of Riyadh's policies, disappeared nearly two weeks ago after entering the Saudi consulate in Istanbul.
Unconfirmed reports said that Khashoggi was likely killed inside the compound, which was denied by Saudi officials as "baseless".
Here is a look at what we know about the disappearance.
What happened to Jamal Khashoggi?
Trump spoke after a personal 20-minute phone call with Saudi King Salman and as the president dispatched his secretary of state to Riyadh for a face-to-face discussion with the king.
Prince Mohammed, King Salman's son, has aggressively pitched the kingdom as a destination for foreign investment.
Mr Khashoggi was known for his interviews and travels with Osama bin Laden between 1987 and 1995, including in Afghanistan, where he wrote about the battle against the Soviet occupation. Saudi Arabia and its close ally, the United Arab Emirates, similarly declared the Brotherhood a terrorist group and arrested its members.
The disappearance of Mr Khashoggi has placed the relationship between the Gulf kingdom and the West under intense strain.
The US signed a almost $110 billion defense deal with Saudi Arabia in May 2017, when Trump made Saudi Arabia a stop on his first foreign trip as president.
Riyadh had so far not allowed the search at the Istanbul consulate - officially Saudi territory - amid reports both sides were at odds over the conditions of the probe. "Would you like to speak up about that?" he said, turning to Sen.
The statement did not directly acknowledge Khashoggi's disappearance, which happened October 2 when he visited the Saudi Consulate in Istanbul.
Ms Cengiz, who spent hours waiting for Mr Khashoggi outside while holding his mobile phones, says her fiance never walked out of the consulate to meet her. Surveillance footage shows her pacing around outside at 5.33pm, speaking on her phone.
Saudis Reportedly Plan To Announce Jamal Khashoggi Was Killed Accidentally
Otaibi took off for Riyadh on a 1400 GMT flight from Istanbul , the Haber-Turk newspaper reported on its website. Pompeo then met a smiling Prince Mohammed, the heir apparent to the throne of the world's largest oil exporter.
What is Turkey saying?.
"It is expected that examinations will be carried out this afternoon", the source said.
On Friday, a source familiar with the ongoing investigation told CNN that Turkish authorities have audio and visual evidence that Khashoggi was killed inside the consulate.
After meeting with Turkish Foreign Minister Mevlüt Çavuşoğlu in London on Monday, UK foreign Secretary Jeremy Hunt said: "The case of Jamal Khashoggi remains deeply concerning".
"It depends on how well they cleaned it up", Prinz told The Associated Press. Turki Aldakhil, the General Manager of the Saudi global news network Al Arabiya, said Riyadh was considering some 30 moves that it may take in response to possible sanctions, and many of those he mentions seem pretty harsh.
Turkish officials have not responded to questions from the AP about Mr Khashoggi's Apple Watch. "Palestine was - and shall remain - on the side of Saudi Arabia", he said. "Certainly our involvement in Yemen with Saudi Arabia will be affected", said Flake, R-Ariz.
The crisis has polarized Saudis, with some blaming the nation's enemies and others concerned about the direction the country is heading under Prince Mohammed. The Qatar-funded satellite network Al-Jazeera has extensively focused on Mr Khashoggi's disappearance over the last week. The government claims that he left the building, however, there is no evidence to support the claim.
What is the USA saying? .
"There are simple procedures, that are part of over 30 others, that Riyadh will implement directly, without flinching an eye if sanctions are imposed", he said.
A delegation from Saudi Arabia arrived in Turkey for the investigation into Khashoggi's disappearance, Turkey's state-run Anadolu Agency reported Friday.
Over the weekend a USA senior administration official said that the U.S. hasn't really dealt much with the Turks on the Khashoggi investigation yet because United States officials, including the embassy staff, mostly focused last week on repatriating American pastor Andrew Brunson, who was accused of plotting a coup attempt against Erdogan.
The three countries also called for Saudi Arabia and Turkey to jointly work on the investigation and for Riyadh to give a detailed response to the murder accusation put forward by Istanbul.