Mattis said he met al-Jubeir during a conference in Bahrain on Saturday and discussed the killing of Saudi jornalist, World News reported.
"We discussed it. You know the same thing we talked about, the need for transparency, full and complete investigation," Mattis told a small group of reporters travelling to Prague with him.
"(There was) full agreement from foreign minister Jubeir, no reservations at all, he said we need to know what happened and it was very collaborative, in agreement," he added.
Saudi Arabia has admitted Khashoggi was killed inside its consulate in the Turkish city of Istanbul.
Khashoggi - a Saudi writer, US resident, and Washington Post columnist - entered the building on October 2 to obtain documentation certifying he had divorced his ex-wife so he could remarry.
After weeks of repeated denials that it had anything to do with his disappearance, the kingdom eventually acknowledged that the murder was premeditated. The whereabouts of his body are still unknown.
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