Then [Curunír] perceived that Sauron also had learned of the manner of Isildur’s en

Then [Curunír] perceived that Sauron also had learned of the manner of Isildur’s end, and he grew afraid and withdrew to Isengard and fortified it; and ever he probed deeper into the lore of the Rings of Power and the art of their forging. ~ The Silmarillion, Of the Rings of Power