A senior Saudi security official who for years was the key go-between for Britain's MI6 and other Western spy agencies in Saudi Arabia is now being persecuted along with his family, according to former Western intelligence officials.
Dr Saad al-Jabri, who helped foil an al-Qaeda bomb plot against the West, fled into exile three years ago, ahead of a purge by the all-powerful Crown Prince Mohammed bin Salman. Now his children have been seized as "hostages", according to his eldest son, Khalid.
"Omar and Sarah were kidnapped at dawn on 16 March and taken out of their beds by about 50 state security officers who arrived in 20 cars," says their brother, Khalid al-Jabri.
The family house in Riyadh was then searched, the CCTV memory cards removed and the pair, aged 21 and 20 respectively, held incommunicado at a detention centre.
There have been no charges given and no reason offered to the family for their arrest, Khalid tells me on a phone call from Canada where he and his father live in self-imposed exile. "We don't even know if they are alive or dead."
He believes they are being held as bargaining chips in an attempt to force his father to return to Saudi Arabia where he fears he will face immediate arrest and imprisonment.
"They can make up any lies they want about him but he is innocent."
The Saudi authorities have not responded to the BBC's requests for comment on the allegations made by the family of Dr Saad al-Jabri and those who worked with him.
Who is Saad al-Jabri?
For years he was the right-hand man, the gatekeeper, to Prince Mohammed bin Nayef, who was widely credited with defeating the al-Qaeda insurgency in the 2000s. He was also the linchpin in all Saudi Arabia's relations with the "Five Eyes" (US, UK, Canada, Australia, New Zealand) intelligence agencies.
In 2010 this crucial link "helped save hundreds of lives", according to a former Western intelligence officer who worked with him.
Al-Qaeda in Yemen had smuggled a powerful bomb on-board a cargo plane bound for Chicago, hidden inside a printer ink toner cartridge. But Saudi intelligence had a human informant inside al-Qaida who provided the tip-off to MI6, even relaying the serial number of the device it was hidden in.
British counter-terrorism police then located and defused the bomb inside the plane at East Midlands Airport. "If that had gone off as planned over Chicago hundreds would have been killed," said the former intelligence officer.
"Dr al-Jabri transformed Saudi counter-terrorism efforts," says another former western intelligence official.
"He changed it from being a crude, violent, confession-based system into one that used modern forensics and computer-based data mining.
"He was the smartest guy we dealt with amongst so many others who were dysfunctional," he says.
A quiet-spoken man with a doctorate in artificial intelligence from Edinburgh University, Dr al-Jabri rose to the rank of cabinet minister and held a major-general's rank in the interior ministry.
But in 2015 everything changed. King Abdullah died and his half-brother Salman ascended to the throne, appointing his young and untested son Mohammed Bin Salman (known as MBS) as defence minister.