According to the statement, the killing of Khashoggi, a Washington Post columnist and a supporter-turned-critic of Saudi Crown Prince Mohamad bin Salman, was "pre-planned and his body parts were disposed off after being dismembered".
The prosecutor's office also noted that no concrete results had come out of the meetings with Saudi Arabia's top prosecutor Sheikh Saud al-Mojeb in Istanbul, "despite the good intentions to uncover the truth behind Khashoggi's killing".
The Istanbul prosecutor's office disclosed the new details in a statement issued soon after the Saudi official, Saud al-Mojeb, flew home after a three-day visit.
The statement, which was distributed to Turkish news channels and read out by reporters on live television, said that Khashoggi was strangled October 2 and his body dismembered and completely destroyed.
Riyadh on Saturday rejected Ankara's request to extradite the Saudis being held over Khashoggi's murder and called on a Turkish delegation to visit Saudi Arabia to present evidence.
File image of Hatice Cengiz, the fiancee of late journalist Jamal Khashoggi.
Mr Khashoggi, a Saudi national working for the U.S. press, was a well-known critic of his home country's rulers.More news: Iran Starts Oil sales to Private Exporters to Beat US Sanctions
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Saudi Arabia has repeatedly shifted its account of what happened to Khashoggi after he entered its consulate nearly a month ago.
"Experts on Saudi Arabia are virtually unanimous in saying that such an audacious mission must have been known about, and most likely was ordered, by the kingdom's de facto ruler, Crown Prince Mohammed bin Salman", it said.
The ruling Justice and Development Party (AK Party) spokesperson Ömer Çelik said Wednesday that Saudi journalist Jamal Khashoggi could not have been killed without orders from high-level positions.
Turkey says a trial in Turkey would be transparent, reflecting concerns about Saudi attempts to dodge responsibility for the killing.
Mattis, in his speech at the conference, said Khashoggi's killing undermines stability in the region, and warned that the US may take additional actions in response to the killing.
Under mounting pressure, Saudi Arabia changed its narrative about Mr Khashoggi's killing several times, eventually admitting that Mr Khashoggi died inside the consulate and only recently acknowledging that Turkish evidence shows his killing was premeditated.