The chief prosecutor's office in Istanbul said that during those meetings, Turkey and Saudi Arabia had "agreed to investigate, search and uncover all details of the event from all angles and to bring those who carried out and planned it to ... justice".
"Despite all our good intentions and efforts to unravel the truth, a concrete outcome was not reached from the meetings", the statement said. It recently acknowledged that Turkish evidence shows the killing was premeditated.
He then flew out of Istanbul on Wednesday afternoon without making a public statement.
"The explanations given so far by Saudi Arabia are not sufficient", Cengiz said.
"Until Saudi Arabia officially announced his death, I never gave up hope that he was alive", Cengiz told BBC Arabic on Monday.
He met Istanbul chief prosecutor Irfan Fidan on Monday and asked to be given the full findings of the Turkish investigation, including all images and audio recordings, Turkish broadcaster TRT reported.
Sources told Sky News that body parts belonging to the writer had been discovered in the garden of the Saudi consul general in Istanbul - located around 500 metres away from the consulate.
"We have requested information from Saudi officials regarding the whereabouts of Jamal Khashoggi's body and the identity of the supposed local collaborator.
In other words, we did not get the impression that they were keen on genuinely cooperating with the investigation", the official said.
Mystery surrounds deaths of Saudi sisters found in NY
In a follow-up statement , the Saudi Arabian consulate described the pair as "students accompanying their brother in Washington". The lack of obvious trauma appeared to rule out a theory they may have jumped into the river from the George Washington Bridge.
Johnson also earns £275,000 a year from his Daily Telegraph column, which he took up after quitting the government over Theresa May's Brexit policy last July.
The case has sorely strained relations between Saudi Arabia and the West, and has fuelled global debate about arms deliveries to the ultra-conservative kingdom.
Erdogan maintains that a 15-strong team traveled from Riyadh to kill Khashoggi, who had been a vocal critic of Saudi Arabia's Crown Prince and de facto ruler Mohammad bin Salman, at the Saudi diplomatic compound in Istanbul.
"This crime has to be punished and the perpetrators identified".
The case has brought near unprecedented global scrutiny on Saudi Arabia and its powerful Crown Prince Mohammad bin Salman, whom Khashoggi had criticized.
Cengiz later told an audience she was disappointed with Trump's response to the killing, suggesting United States business ties with Riyadh had influenced his approach to the case. (AP) Donald Trump said Khashoggi's death was the worst cover up in history.
The country denies the ruling royal family was involved.
However relations between the long-time allies have cooled after the murder and yesterday Washington called for a ceasefire and peace talks in Yemen, where the United States has faced fierce worldwide criticism for supporting a Saudi-led coalition.
"I and my government would like all those responsible, from the person who gave this order to those who carried it out, to be brought to justice and punished under worldwide law", she said.