Mattis told reporters traveling with him that Saudi Foreign Minister Adel al-Jubeir "had no reservations at all" about the need for the kingdom to be transparent about the death of the Washington Post columnist at the kingdom's consulate in Istanbul, Turkey.
During a press conference with Czech Prime Minister Andrej Babis later Sunday, Mattis added that the evidence Turkey has collected "will ensure there is more than one review" of the incidents surrounding Khashoggi's death.
He and Jubeir spoke privately at a dinner Saturday during the Manama Dialogue international security conference in Bahrain. Mattis did not detail the conversation they had but said he told Jubeir that "we need to know what happened."
Turkish officials have said that a Saudi team of 15 men tortured, killed and dismembered the writer in a premeditated act. The kingdom initially said it knew nothing about what happened to Khashoggi, but on Thursday said evidence shows the killing was premeditated.
When asked about the killing during a panel discussion at the conference, Jubeir complained about the media "hysteria" and the rush to pin blame on the Saudi royal family before the investigation is complete.
Khashoggi lived in self-imposed exile in the U.S. for the past year and wrote editorial columns for The Washington Post that were critical of Saudi Crown Prince Mohammed bin Salman, the kingdom's heir apparent.
Mattis, in his speech at the conference, said Khashoggi's killing undermines stability in the region, and warned that the U.S. may take additional actions in response to the killing. He did not say what those steps would be.