Saudi Arabia's heir to the throne, Crown Prince Nayef bin Abdel-Aziz al Saud, has died in hospital.
The royal family said in a statement that Nayef, 79, died abroad. They did not specify where.
According to experts on the Saudi monarchy, Nayef was treated abroad in April for cancer.
Nayef, who was also the kingdom's hardline interior and vice-prime minister, was named as crown prince last year.
He earned praise in the West for leading crackdowns on al Qaeda in Saudi Arabia, which was home to 15 of 19 of the September 11 hijackers.
But he was harshly criticised for a 2002 interview in which he said "Zionists" benefitted from the 9/11 attacks because it turned world opinion against Islam and Arabs.
He also opposed some moves by King Abdullah, his half-brother, for more openness in the strictly conservative society.
Nayef's death unexpectedly reopens the question of succession in this crucial US ally and oil powerhouse for the second time in less than a year.
King Abdullah, 88, has now outlived two designated successors, despite ailments of his own.
Now a new crown prince must be chosen from among his brothers and half-brothers, all the sons of Saudi Arabia's founder, Abdul-Aziz.
Nayef, a soft-spoken man, was born in 1933, the 23rd son of Abdul-Aziz.
He was one of the five surviving members of the Sudairi seven, sons of Abdul-Aziz from his wife Hussa bint Ahmad Sudairi who have held influential posts for decades.