Turkish media outlets have published CCTV footage which they say shows evidence of a plot linked to missing Saudi journalist Jamal Khashoggi.
It shows men said to be Saudi intelligence officers entering and leaving Turkey via Istanbul airport.
Mr Khashoggi, a US resident and critic of the Saudi monarchy, entered the Saudi consulate in Istanbul on 2 October and has not been seen since.
Turkish authorities say Mr Khashoggi was killed. Saudi Arabia denies this.
On Wednesday US President Donald Trump said he wanted to "get to the bottom" of the case, adding: "We cannot let this happen to reporters, to anybody."
He said the situation was "serious" and wished to invite Mr Khashoggi's fiancée, Hatice Cengiz, to the White House.
What else does the video show?
Broadcast by Turkey's TRT World channel and apparently garnered from security cameras, the footage shows vehicles driving up to the consulate, including black vans thought to be central to inquiries.
Groups of Saudi men are seen entering Turkey via Istanbul airport, checking in at hotels and later leaving the country.
Turkish investigators are looking into two Saudi Gulfstream jets that landed at the airport on 2 October. The video shows aircraft waiting on the tarmac.
Mr Khashoggi was visiting the consulate to finalise his divorce so he could marry Ms Cengiz.
He is seen on the video entering the consulate. His fiancée waits outside.
Turkey's Sabah newspaper reports that it has identified 15 members of an intelligence team it says was involved in the Saudi's disappearance. Among them was a forensics expert, it says.
The BBC's Mark Lowen says one of the men was once posted to London.
These are the 15 Saudis that Turkish pro-govt paper @Sabah says flew to Istanbul, allegedly as #Khashoggi hit squad: https://t.co/ieXfaQadJo— Mark Lowen (@marklowen) October 10, 2018
I’ve been told fourth from last picture, Maher Mutreb, in black jacket, was a colonel in #Saudi intelligence based at its embassy in London
Police are reported to be examining 150 security cameras as part of their investigation.
Some local media have also been reporting that Mr Khashoggi may have been abducted rather than killed.
Turkey says it will conduct a search of the Istanbul consulate, while Saudi Arabia's foreign ministry said the country was "open to co-operation" and a search of the building could go ahead.
Ankara is demanding that Saudi Arabia prove Mr Khashoggi left the building, while not providing definitive evidence to support the claim he was killed inside.
Who is Jamal Khashoggi?
A critic of Crown Prince Mohammed bin Salman, Mr Khashoggi was living in self-imposed exile in the US and writing opinion pieces for the Washington Post before his disappearance.
A former editor of the al-Watan newspaper and a short-lived Saudi TV news channel, he was for years seen as close to the Saudi royal family. He served as an adviser to senior Saudi officials.
But after several of his friends were arrested, his column was cancelled by the al-Hayat newspaper and he was allegedly warned to stop tweeting, Mr Khashoggi left Saudi Arabia for the US.
What has the international reaction been?
The UK's foreign secretary has told Saudi Arabia that Britain expects urgent answers over the disappearance of Mr Khashoggi.
In a phone call to Saudi Foreign Minister Adel al-Jubeir, Jeremy Hunt warned that "friendships depend on shared values".
US Secretary of State Mike Pompeo called on Saudi Arabia to "support a thorough investigation" of his disappearance and "to be transparent about the results".
UN experts have demanded a "prompt independent and international investigation" into his disappearance.
Last week, Saudi Crown Prince Mohammed told Bloomberg News that his government was "very keen to know what happened to him", and that Mr Khashoggi had left "after a few minutes or one hour".
Crown Prince Mohammed's brother and the Saudi ambassador to the US, Prince Khaled bin Salman al-Saud, has insisted all the reports about his disappearance or death "are completely false and baseless".
Campaign group Reporters without Borders has called for an independent international investigation into Mr Khashoggi's disappearance.
The group says more than 15 journalists and bloggers have been arrested in Saudi Arabia since September 2017.
How has Khashoggi's fiancee reacted?
Hatice Cengiz has appealed to the US for help in an emotional article in the Washington Post.
"I implore President Trump and First Lady Melania Trump to help shed light on Jamal's disappearance," she wrote.
"We were in the middle of making wedding plans, life plans," when he vanished, she said.
"Jamal is a valuable person, an exemplary thinker and a courageous man who has been fighting for his principles. I don't know how I can keep living if he was abducted or killed in Turkey."