The officers began searching a two-storey building in Termal district in Yalova with the help of sniffer dogs and drones before the investigation widened to the adjacent villa, state news agency Anadolu said.
The news outlet, without citing a source, said the villas belong to two Saudi businessmen, including one who had allegedly been contacted by phone by some members of the 15-man Saudi assassination squad that traveled to Istanbul to carry out the deadly operation.
Anadolu did not name the businessman but said he was outside of Turkey at the time of the murder and had not returned in the past two months.
The focus of the search was a well on the grounds of the first villa, which was being drained of water with special equipment brought to the scene.
Khashoggi, a prominent commentator on Saudi affairs who wrote for the Washington Post's Global Opinions section, had lived in self-imposed exile in the USA since September 2017, when he left Saudi Arabia over fears of the Riyadh regime's crackdown on critical voices.
Investigators descended on the mansion and a neighbouring property in the province of Yalova, south of Istanbul, as part of their ongoing search for the body of the murdered Washington Post columnist.
Police previously searched areas in Yalova as well as around Istanbul, including the Saudi Consulate, the nearby residence of the Saudi consul general and a forest on the outskirts of the city. Turkish officials have detailed what they say is evidence of Saudi planning - including the scouting of locations where Khashoggi's body could be dumped - as well as extensive attempts by Saudi agents to cover up the crime. Khashoggi was a critic of Saudi Crown Prince Mohammed bin Salman.
The kingdom initially said Khashoggi had walked out of the consulate before shifting its account of what happened amid Turkish intelligence leaks.
Ankara insists the orders for the killing came from the highest levels of the Saudi government, but not King Salman.
His body has not been found and his death has triggered widespread global criticism of Saudi Arabia.
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