"The victim's body was dismembered and destroyed following his death by suffocation - again, in line with advance plans".
The Washington Post journalist was killed in the kingdom's Istanbul consulate earlier this month. The newspaper claimed that his fingers were cut off and that he was killed by being beheaded.
"Then it should take decisive action to impose sanctions on those responsible - including, if the available evidence points to him, Mohammed bin Salman - and reshape US relations with Saudi Arabia", said the board.
Cengiz last week declined an invitation by the White House, criticizing Trump's statements on Khashoggi's killing.
Turkish President Recep Tayyip Erdogan says his country will reveal more evidence about the killing but is not in any rush to do so.
It came at the end of a three-day visit by Saud al-Mojeb to Istanbul to hold talks with Fidan and other Turkish officials.
Turkey says the kingdom's top prosecutor, Saud al-Mojeb, is expected to arrive on Sunday.
Washington Post columnist Khashoggi's murder has escalated into a crisis for the world's top oil exporter as Saudi Arabia's allies have reacted with outrage.
"Support Turkey's Efforts": Jamal Khashoggi's Fiancee Urges Trump
It's a month to the day since Jamal Khashoggi walked into the Saudi consulate in Istanbul - and was never seen again. Finally, on October 25, Saudi Arabia said the murder was " premeditated " based on information supplied by Turkey .
Turkey's foreign minister, Mevlut Cavusoglu, on Monday welcomed the cooperation between Turkish and Saudi investigators and said he hoped there would be no further delays.
Ankara also demanded extradition of all suspects in this case, arguing that the journalist was killed on the Turkish territory and the investigation of this case thus falls within the jurisdiction of the Turkish authorities.
"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 traveling to Prague with him.
Celik added: "We are not blaming anyone in advance but we will not allow anything to be covered up".
The slain journalist had also been an outright critic of the powerful Crown Prince Mohammed bin Salman and his policies.
Riyadh initially insisted that Khashoggi left the consulate unharmed, but as pressure grew, Saudi state media changed the story and said Khashoggi died when an argument descended into a brawl. Its authorities remain frustrated at a perceived lack of co-operation by Saudi Arabia, according to the BBC's Mark Lowen in Istanbul.
After releasing the results of its initial investigations, the Saudi Public Prosecution announced that 18 people were arrested over their alleged connections to the killing.