The case has brought near unprecedented worldwide scrutiny on Saudi Arabia, which is seeking to draw a line under the crisis after offering a series of differing narratives in the weeks following Mr Khashoggi's murder.
Khashoggi's death inside the kingdom's Istanbul consulate has brought near unprecedented global scrutiny on Saudi Arabia, which is seeking to draw a line under the crisis as Western powers demand answers.
When asked who was ultimately responsible for the killing, Ms. Cengiz told Reuters in an interview in Turkish: "This took place inside a Saudi diplomatic mission ..."
At the same conference, Mattis had sharp words for Saudi Arabia, saying that the killing undermined Middle Eastern stability and that Washington would take additional measures against those responsible.
"I am sure the Saudi regime knows where his body is".
The fiancée of Jamal Khashoggi on Monday criticized President Donald Trump's response to his killing, urging him to set aside USA trade interests in the push for truth, and demanded Riyadh disclose more details to bring those who ordered it to justice.
Slain journalist Jamal Khashoggi was about to disclose some detailed information about Saudi Arabia's use of chemical weapons in Yemen and the United Kingdom had been made aware of a planned plot against him weeks before he was killed in the Saudi Consulate building in Istanbul, a British tabloid claimed on Sunday.
Ms Cengiz, who was waiting outside, raised the alarm after her fiancé didn't reappear after ten hours.
Cengiz said she had not been contacted by Crown Prince Mohammed or the Saudi Royal family, nor offered any condolences by them.
Germany, which has pledged to suspend arms exports to Saudi Arabia over the killing of journalist Jamal Khashoggi, yesterday said it expects the European Union to adopt a common position on potential limits to weapons sales.
Top Saudi and Turkish prosecutors on Monday discussed the investigation into the killing of Saudi writer Jamal Khashoggi, a show of cooperation despite differences over which country should try 18 suspects who were detained in Saudi Arabia.
"President Trump should help reveal the truth and ensure justice be served", Ms Cengiz said through a translator.
He said the culprits were in Saudi Arabia and the country had a big responsibility in this regard.
He clarified later that the murder and cover-up should have never happened.
US Defence Secretary Jim Mattis said Sunday that Jubeir had assured him Riyadh would conduct a "full" investigation, adding he was confident the probe would include Turkey's findings. "Let's not let money taint our conscience and compromise our values", she told a memorial in London for the slain Washington Post columnist. "I don't want to just trust the Turks or the Saudis".
They say they have audio and video evidence to support this claim.
Turkish law required that Khashoggi, who was divorced, provide proof that he did not have a wife and thus his trip to the consulate.
Reports in the Turkish media have said that Mr Khashoggi's body, which has still not been found, was cut up into multiple pieces.