He visited the consulate in Istanbul on October 2 to obtain documentation to finalize his divorce; his Turkish fiancée waited for him outside the building, and reported him missing after he didn't return.
Turkish authorities say Mr Khashoggi was killed.
However, Saudi Arabia has offered no evidence to support its contention that the writer left the consulate unharmed and vanished into Istanbul while his fiancée waited impatiently outside.
The alleged involvement of a forensics expert adds weight to the suspicions.
Saudi officials meant to lure Khashoggi back to Saudi Arabia, the Washington Post reported the source as saying, but it was unclear what they meant to do with him - and whether the USA ever warned Khashoggi of the threat he faced. Saudi Arabia claimed that Khashoggi had left the embassy without anyone noticing.
Erdogan's comments were his most direct suggestion yet about potential Saudi culpability in Khashoggi's disappearance.
He has not been heard of since.
Neil Quilliam, a senior research fellow for the Middle East and North Africa at the Chatham House think tank, said the Saudis were sure to notice when Trump said he knew "nothing" about Khashoggi's case.
"When President Trump became president, we've changed our armament strategy again for the next 10 years to put more than 60% with the United States of America", he added.
The source said they were told Mr Khashoggi is still alive, contradicting the claims he was murdered in the Istanbul consulate. But he also criticised Turkey's crackdown following a 2016 failed coup against President Recep Tayyip Erdogan.
Among those who evinced little serious concern for Khashoggi's wellbeing was President Donald Trump. "But if this deeply disturbing news report is confirmed, the United States & the civilized world must respond strongly, and I will review all options in Senate".
IMF, Pakistani officials to meet today as govt seeks bailout
The government now will have to increase regulatory duties (RD) and customs duty (CD) on imports to contain trade deficit. It now expects the global economy to only expand by 3.7% in 2018 and 2019, down from 3.9% before.
The interview was conducted three days before Khashoggi's disappearance, when he was in London for a conference, the BBC said.
The journalist said he had been banned from writing in the pan-Arab Al-Hayat newspaper, owned by Saudi prince Khaled bin Sultan al-Saud, over his defence of the Muslim Brotherhood which Riyadh has blacklisted as a terrorist organisation. Associated Press writer Walter Ratliff in Washington contributed.
Turkish authorities have been given access to search Saudi Arabia's consulate in Istanbul as part of an investigation into a missing dissident journalist.
"The condition of the lost journalist, details on him and who is responsible for this will be uncovered", Omer Celik said.
Khashoggi was previously a prominent newspaper editor in Saudi Arabia and an adviser to a former head of intelligence.
"To do otherwise would betray those who languish in prison".
The Washington Post, the daily to which Khashoggi was a contributor, reported that Saudi Crown Prince Mohammed bin Salman ordered an operation to "lure" the critical journalist back home.
She said Mr Khashoggi had been required to surrender his mobile phone, which is standard practice in some diplomatic missions.
"Deeply troubled to hear reports about Saudi Arabian journalist Jamal Khashoggi".
He noted 47 U.S. senators recently voted to ban arms sales to Saudi Arabia - four short of a majority.
Opening cupboards, filing cabinets and wooden panels covering air conditioning units, Otaibi walked through the six floors of the building including a basement prayer room, offices, visa counters, kitchens and toilets as well as storage and security rooms. Police also said a special team of around 15 Saudis were especially sent to Istanbul and in the building at the same time as Khashoggi.