Khashoggi, 59, went missing while on a visit to the consulate in Istanbul for paperwork to marry his Turkish fiancée.
Trump declined to say whom he talked with in the Saudi government.
Turkish officials have accused Saudi officials of flying in a 15-person hit team to kill and dismember Khashoggi before flying his body out of the country within hours of his arrival at the consulate.
"I have to find out what happened", he said.
As a contributor to the Post, Khashoggi has written extensively about Saudi Arabia, including criticising its war in Yemen, its recent diplomatic spat with Canada and its arrest of women's rights activists after the lifting of a ban on women driving.
"We demand from the global community to pressure Saudi Arabia and Mohammed bin Salman to tell us exactly what happened".
Last year, Khashoggi left Saudi Arabia in fear of retribution, and he's been living in self-imposed exile in the U.S.
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Khabib's father and trainer, Abdulmanap Nurmagomedov, would be a dream match-up for him, which might factor in his son's thinking. McGregor (21-4) suffered a fourth-round submission loss to Nurmagomedov (27-0) at UFC 229 last weekend in Las Vegas .
"We call for cooperation between Turkey and Saudi Arabia to conduct a prompt and impartial investigation into the circumstances of Mr. Khashoggi's disappearance and to make the findings public", United Nations human rights spokeswoman Ravina Shamdasani told reporters in Geneva. "As we speak today, there are Saudi intellectuals and journalists jailed".
While a military conflict is not likely to arise between Turkey and Saudi Arabia, this new development will place more strain on the relationship, further complicating the already hard situation in the Middle East.
"Is it possible for there to be no camera systems at the Saudi Arabia consulate, where the event took place?", Erdogan was quoted as saying by Hurriyet. "We hope that he is safe and that we can hear from him soon".
Turkey has asked for permission to search Saudi Arabia's consulate in Istanbul, broadcaster NTV said on Monday. "Don't you have cameras and other things?"
A Turkish diplomatic source confirmed the Saudi envoy was "invited" to the ministry on Sunday in Ankara and was met by deputy foreign minister Sedat Onal.
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Bush , Barbara Bush , Craig Coyne , Barbara Bush and Poppy Louise Hager at Barbara and Craig's wedding on October 7. Following this experience, graduating from Vanderbilt University and serving as a Page in the U.S.
She said she waited outside for him to come out and then realized something was wrong around 4 p.m. because the consulate closed at 3:30 p.m.
The relationship between Saudi Arabia and Turkey has always been contentious.
Secretary of State Mike Pompeo on Monday called on Saudi Arabia to investigate the odd disappearance of journalist Jamal Khashoggi, whose case has sparked worldwide outcry over the extent of the kingdom's crackdown on dissent.
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Barcelona , on the other hand, have just three candidates, Lionel Messi , Luis Suarez and Ivan Rakitic. The victor will be announced in Paris on 3 December with a women's victor awarded for the first time.