The prosecutor has charged 11 people over the murder and is seeking the death penalty for five of them.
Saudi Arabia's public prosecutor has concluded a senior intelligence officer ordered Jamal Khashoggi's murder, and not Crown Prince Mohammed bin Salman.
He added that Khashoggi's body was dismembered and then transferred outside the Saudi consulate building.
After first insisting Khashoggi left the consulate unharmed, Saudi authorities said he was killed in an argument that degenerated into a brawl before finally accepting what Turkey had said virtually from the start - that he was killed in a premeditated hit.
Saudi agents originally wanted to bring Khashoggi back to Saudi Arabia alive, the spokesman said.
"The crime included a fight and injecting the citizen [Khashoggi] with a drug overdose that led to his death", the official said. The adviser believed that Khashoggi had been co-opted by organizations and countries hostile to the kingdom "and his presence overseas represented a danger to national security", the prosecutor said.
The five people facing capital punishment were directly involved in "ordering and executing the crime", Saud al-Mojeb said at a news conference in Riyadh, where he also exonerated Crown Prince Mohammed bin Salman in the killing. But on Thursday, the prosecutor said he believes "the head of the negotiation team chose to murder the victim if the negotiations [for his return to Saudi Arabia] failed".
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A former Saudi adviser was also involved in the planning of the murder, the spokesperson said, adding that the head of the team tasked with convincing the journalist to return to the Kingdom was also the person who made the decision to murder him, the report stated.
It said a total of 21 individuals were in custody in connection with the killing, 11 of whom have been indicted with investigations to continue into the others. The head of the negotiation team chose to murder the victim if the negotiations failed.
On Thursday, Turkish Foreign Minister Mevlut Cavusoglu reiterated his country's demand that the men should be tried in Turkey.
U.S. President Donald Trump had suggested ultimate responsibility lay with the prince as de facto ruler.
The journalist went into self-imposed exile in the United States in 2017 after falling out with Prince Mohammed. "We are waiting for a response", the prosecution said.
Khashoggi's killing has provoked global outrage but little concrete action by world powers against Saudi Arabia, the world's largest oil exporter and a supporter of Washington's plans to contain Iranian influence across the Middle East.