Now the office of the Chief Istanbul Prosecutor Irfan Fidan says Khashoggi was strangled as soon as he entered the diplomatic compound as part of a premeditated killing.
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".
Khashoggi - a fierce critic of Riyadh's human rights violations and of Crown Prince Mohammed bin Salman's policies - disappeared after entering the kingdom's consulate in Istanbul on October 2 for the paperwork needed for his planned marriage.
After this, the statement alleges, Saudi officials proceeded with dismembering Khashoggi to secretly transport his body and cover up the killing.
An investigation into the role of the Saudi leadership in the death of Khashoggi would also undermine the 15-month-old economic and diplomatic boycott of Qatar, which Saudi Arabia, the UAE, Egypt, and Bahrain accuse of supporting terrorism. It said it has arrested 18 suspects, who will be prosecuted in Saudi Arabia.
The president has also asked Saudi officials to disclose who ordered the killing of Khashoggi and sent the team to Istanbul.More news: Qualifier Copil stuns Zverev to make final: Federer wins
Turkish officials had previously revealed details about the October 2 death of Khashoggi, including the strangling and dismemberment but always anonymously and usually through leaks to the Turkish media.
The crown prince, 33, is widely blamed for Washington Post columnist Mr Khashoggi's death at the kingdom's consulate in Istanbul, Turkey, at the hands of a Saudi hit squad.
Turkey has not publicly blamed Saudi Arabia for the murder.
United States President Donald Trump has also come under vast pressure for his support for Riyadh, which is a key cog to the US' interests in the Middle East. The Turkish authorities had released a video evidence of him entering the Saudi consulate in Istanbul.
"Yesterday, our prosecutor told the Saudi prosecutor that the prosecution could be carried out in Turkey since the location of the crime is Istanbul", Erdogan said at Turkey's parliament.
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.