The person spoke on the condition of anonymity because he is not authorized to speak on behalf of the administration or the Saudi government. He also said he thought third parties were involved in the death.
An undisclosed number consulate employees in Istanbul were interviewed by prosecutors, the semiofficial Anadolu news agency reported, a day after Turkish authorities began combing through wooded areas outside Istanbul in an apparent search for Khashoggi's remains.
The Saudi prosecutor has said that 18 suspects have been taken into custody so far.
The state media also reported that two high-ranking Saudi officials have been fired by King Salman, including Maj.
Deputy intelligence chief Ahmad Al Assiri and royal court media adviser Saud Al Qahtani, both part of Crown Prince Mohammed bin Salman's inner circle, were sacked.
King Salman, his father, appointed him to lead a committee that will restructure the kingdom's intelligence services after Mr Khashoggi's slaying.
Investigating Turkish officials surmised that Khashoggi was killed during his time in the building, an allegation that Saudi Arabia initially denied. According to numerous reports citing Turkish officials familiar with the probe, the journalist was ambushed by Saudi agents, cruelly tortured and murdered.
Mr Khashoggi's fiancée, Hatice Cengiz, questioned in a tweet what had happened to his body, saying: "The heart grieves, the eye tears, and with your separation we are saddened, my dear Jamal".
Trump said the death was "unacceptable" and there would be a response from the USA, but that he would wait to speak to the crown prince before announcing the U.S. response.
The admission - after persistent claims by the Saudi authorities that Khashoggi had left the consulate alive - came amid the threat of USA sanctions.
Earlier this week, unnamed Turkish officials told media outlets they had audio and visual evidence to prove this. "We will continue to closely follow the global investigations into this tragic incident and advocate for justice that is timely, transparent and in accordance with all due process", said White House spokesperson Sarah Sanders.More news: Google to charge smartphone makers for Google Play in Europe
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October 15: almost two weeks after Khashoggi's disappearance, teams of Turkish investigators enter the consulate to start their search. "The brawl led to his death and their attempt to conceal and hide what happened".
It took Khashoggi seven minutes to die, the sources said.
Additionally, 18 Saudi nationals were detained after the initial investigation findings.
French Foreign Minister Jean-Yves Le Drian also demanded an "exhaustive and diligent investigation" into the death of the journalist.
Saud Qahtani, a powerful adviser to Prince Mohammed, also was sacked.
Arab countries have similarly not commented on the affair.
"I will remain a loyal servant to my country for all times", he wrote.
A protester outside the Saudi Arabian consulate in Istanbul earlier this month holding a portrait of Jamal Khashoggi with the caption "Jamal Khashoggi is missing since October 2".
The announcement could ease strains with the USA and Turkey but leaves lingering questions about the extent of Prince Mohammed's role in and knowledge of the incident.