Meanwhile, Turkish President Recep Tayyip Erdogan has said Saudi Arabia must prove Khashoggi left their building alive. But after writing critically on the crackdown following crown prince Mohammed bin Salman's rise to power, he left Saudi Arabia in self-imposed exile for the US, where he became a regular commentator on Saudi affairs and foreign policy for The Washington Post.
The surveillance image released by The Washington Post bore a date and time stamp, as well as a Turkish caption bearing Khashoggi's name and that he was arriving at the consulate.
"We wouldn't normally broadcast an off-air conversation, but we've chose to make an exception, in light of the current circumstances", the British broadcaster said in a note published with the recording.
But Dalay, a non-resident fellow at Brookings Institution, said Turkish officials do not want the case to be seen as "politicised from the start". They checked into two hotels in Istanbul, the paper said.
The Committee to Protect Journalists said it was alarmed by reports Khashoggi may have been killed inside the consulate.
The US president, Donald Trump, a robust ally of Saudi Arabia, who had pledged to stay out of the country's domestic affairs, made his first reference to the case on Monday, telling reporters he was "concerned" about "some pretty bad stories" about Khashoggi's fate.
The Saudis praised "the brotherly Turkish government" for accepting a request to send a "security delegation of Saudi investigators" to assist in the inquiry into Khashoggi's disappearance.
Khashoggi had entered the consulate to get documents for his forthcoming marriage.
Saudi Arabia calls Khashoggi murder claims 'lies'
He added that investigations are still ongoing.
The van, which had diplomatic number plates, left in a convoy around two hours after Khashoggi entered. "He writes out of a sense of love for his country and deep faith in human dignity and freedom".
Turkish Foreign Ministry spokesman Hami Aksoy said the Vienna Convention allowed for consulates to be searched by the authorities of the host country with the consent of the mission chief.
Khashoggi, 59, went missing while on a visit to the consulate in Istanbul for paperwork to marry his Turkish fiancee.
His disappearance has sparked global concerns, particularly after Turkish sources said over the weekend that authorities believed he had been killed inside the consulate.
Khashoggi had sought to become a USA citizen after living in self-imposed exile since a year ago, fearing repercussions for his criticism of the prince, Cengiz wrote.
The journalist has been a vocal critic of Riyadh's intervention in the war in Yemen, and once compared the young prince to Russian President Vladimir Putin in a Washington Post column.
Rights office spokeswoman Ravina Shamdasani said it would be "truly shocking" if reports of Khashoggi's death are confirmed. Their ambassador to the USA insists that he personally had a friendly relationship with Khashoggi and remained in contact whenever both of them were in Washington.
More than 20 Republican and Democratic senators instructed Trump to order an investigation into Khashoggi's disappearance under legislation that authorizes imposition of sanctions for perpetrators of extrajudicial killings, torture or other gross human rights violations.
"If true, this would be an abysmal new low".