Speaking later on Sunday at a joint news conference in Prague with Czech Prime Minister Andrej Babis, Mattis was asked how an investigation would be able to determine whether Saudi Arabia's powerful Crown Prince Mohammed bin Salman had any involvement in the killing.
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 ..."
In response to the killing, Mattis said Saturday, the administration is pursuing "twin imperatives" of "protecting America and holding accountable those responsible for this murder".
Reports in the Turkish media have said that Mr Khashoggi's body, which has still not been found, was cut up into multiple pieces.
"They need to explain what happened".
He did not walk out of the consulate and Ms Cengiz, who was waiting outside, first raised the alarm.
"No foreign leader who is not enslaved to Saudi Arabia financially would dare to be seen standing next to him", Whitson said.
Turkey hosts summit to seek solutions to end war in Syria
The UN envoy to Syria Staffan de Mistura also attended the summit and discussed the matter of forming a constitutional committee. The villages are part of the demilitarised zone in Idlib , agreed by Turkey and Russian Federation in Sochi last month.
"I and my government would like all those responsible, from the person who gave this order to those who carried it out, to be brought to justice and punished under global law", she said.
In the meantime, Saudi Arabia and Turkey continue to feud over whether 18 suspects in Khashoggi's death will be handed over to Ankara.
Turkish officials say a "hit squad" of Saudi agents dispatched from the gulf killed Khashoggi in the consulate and dismembered his body before returning to Saudi soil.
Turkish President Recep Tayyip Erdogan has said that a 15-person team came from Riyadh to kill Mr Khashoggi.
Macron further said, "It will depend on the facts as they are established and the sanctions will be taken at a European level, as we usually do, so that there is true coordination", according to Al Jazeera. "I want to know the details of who is responsible".
While the U.S. has condemned Khashoggi's killing - Trump called it "one of the worst in the history of cover-ups" - the president and his top aides have repeatedly signaled that the broader U.S. -Saudi relationship, including billions of dollars in military sales, shouldn't be put at risk over the journalist's death.
"The president met last week with CIA Director Gina Haspel after having gathered additional intelligence on her overseas trip and the administration is weighing different options", Sanders told reporters.
President Donald Trump has said little about Saudi Arabia's role in the killing of Khashoggi, a USA resident and insider-turned-critic of the kingdom, since being briefed last week by CIA Director Gina Haspel.