Foreign Minister Adel al-Jubeir called the worldwide outrage over the killing "fairly hysterical" and said that once the Saudi investigation was complete, the suspects would be held accountable "in Saudi Arabia".
Germany has already said it will stop exporting weapons to Saudi Arabia until the circumstances surrounding Mr Khashoggi's death are cleared up.
Trump has given mixed messages over the case, vowing a severe response but also saying that the United States wanted to preserve its close relationship with Saudi Arabia.
Khashoggi was last seen entering the Saudi consulate in Istanbul on 2 October to complete paperwork for his marriage to his Turkish fiancee.
Ms Cengiz said she believes the Saudi regime knows where her late fiance's body is and that she wants the role of the "political leadership in this brutal killing to be brought to light".
The Washington Post reported in 2016 on the use by Saudi Arabia of USA -supplied white phosphurus so it's not clear just what Khashoggi would have been exposing if he, indeed, was about to obtain documentary evidence.
Al-Mojeb, the prosecutor visiting Istanbul, was named attorney general by Saudi King Salman past year after a palace shakeup that saw Prince Mohammed sideline his elder, more experienced cousin, Mohammed bin Nayef, for the title of crown prince. No details were provided following the meeting.
"We discussed it. You know the same thing we talked about, the need for transparency, full and complete investigation", Mattis said while mentioning that the leaders had met at a conference in Bahrain on Saturday. "Full agreement from FM Jubeir, no reservations at all", the Pentagon chief told reporters on a flight from Manama to Prague where he will mark the centenary of Czechoslovakia. "I am sure the investigation will include the evidence that Turkey has put forward so far".
He said: "When I asked him why he was anxious, he didn't really want to reply, but eventually he told me he was getting proof that Saudi Arabia had used chemical weapons".
Turkish President Tayyip Erdogan, who Ms. Cengiz praised, has urged Riyadh to disclose who ordered the murder and prosecutors have prepared an extradition request for 18 suspects from Saudi Arabia. Ms. Cengiz said she had been shown no recordings of what happened inside the consulate and that she got most of her information from the media.
The murder, which has tarnished the image of Crown Prince Mohammed, has sparked a wave of worldwide criticism and affected Washington's relations with the kingdom.
Erdogan has expressed scepticism about Saudi's shifting explanations of the killing and has called on the kingdom to hold to account all those responsible, no matter how highly placed.