The scope of a new roundup of Saudi royals widened on Saturday with word that a fourth senior prince has been detained under orders from Crown Prince Mohammed bin Salman, according to two Saudis close to the royal family, in a sign that he is determined to crack down on even whispers of dissent.

The wave of arrests has ensnared a former head of army intelligence, Prince Nayef bin Ahmed, as well as at least three other senior princes, all detained on Friday. The full extent of the roundup is still not clear.

Crown Prince Mohammed, 34, has already consolidated his power as the de facto ruler of the kingdom in the name of his aging father, King Salman, 84. But the arrests offered new evidence of how far the crown prince would go to lock down potential opponents within his family, stirring new fear within its ranks, according to several people close to the family.

The most senior family member detained was Prince Ahmed bin Abdulaziz, the father of Prince Nayef and the last surviving full brother of King Salman. The arrests of both father and son stunned the royal family because Prince Ahmed’s closeness to the king had so far appeared to provide him a measure of immunity against the wrath of the crown prince, even when he came down on others.