The eldest son of the murdered Saudi journalist, Jamal Khashoggi, has arrived in the US from Saudi Arabia, with family members.
Salah Khashoggi had previously been unable to leave Saudi Arabia due to his father's criticism of the leadership.
Jamal Khashoggi was murdered in the Saudi consulate in Istanbul three weeks ago, investigators say.
Saudi Arabia's public prosecutor is travelling to Turkey on Sunday as part of the investigation.
Turkish President Recep Tayyip Erdogan announced the move in a televised speech, adding that the prosecutor would meet Istanbul's prosecuting authorities.
Turkey also had other evidence, which had not yet been disclosed, Mr Erdogan said.
And he called on Saudi Arabia to disclose the location of Jamal Khashoggi's body, as well as the name of an alleged "local co-operator" involved in disposing of it.
The Saudi prosecutor, Sheikh Saud al-Mojeb, said on Thursday that the killing was premeditated, in a further development of the Saudi narrative.
Saudi officials have also blamed "rogue" agents as Riyadh has struggled to provide a credible explanation for how Jamal Khashoggi met his end.
At first Saudi Arabia maintained the journalist, who had US residency and contributed to the Washington Post, had left the building "unharmed" on 2 October – then that he had been killed in a brawl.
Turkish investigators, speaking off the record, have said Saudi Arabia's de facto leader, Crown Prince Mohammed bin Salman, must have known what was going on. Riyadh denies he had a role in the death.
Two days before arriving in the US, Salah Khashoggi was pictured receiving condolences from Crown Prince Mohammed.
He briefly shook hands with Crown Prince Mohammed and gave what many see as a cold stare during Tuesday's apparently staged meeting.
Some are suggesting the public display may have been the price for being allowed to leave the country.
A US spokesman said that Secretary of State Mike Pompeo had told Saudi leaders that he wanted Salah Khashoggi, a dual citizen, to be able to return to the US and was "pleased" that he had been allowed to do so.
CIA director Gina Haspel has been in Turkey being briefed by investigators. On her return, she briefed President Donald Trump on Thursday.
No details were released, but media reports said that during the Turkey visit Ms Haspel had heard audio recordings that investigators claim capture the murder.
Mr Pompeo was also at the White House with Ms Haspel.
On Wednesday Turkey's Sabah newspaper said she had listened to audio recordings of Khashoggi's interrogation and death, but gave no details about the contents or how the audio had been obtained.
Sources quoted by the Washington Post and Reuters news agency later backed up the reports.
A person "familiar with the audio" told the Washington Post that it was "compelling".
BBC
