Regalus and Voldemort's Horcruxes?

Question

The only people who know about the Voldemort's Horcruxes are: Harry, Dumbeldore, Hermoine, Ron and Scrimgeor. So how did Regalus know about those Horcruxes?

Answer 1

Regulus knew because Voldemort asked him if he could borrow Kreacher. He then took Kreacher with him when he hid the locket in the cave, forcing Kreacher to drink the potion to hide it. Voldemort then left Kreacher in the cave to die, but Kreacher didn't succumb to the effects of the potion, he managed to disapparate back to Grimmauld Place and told Regulus what had happened. Regulus was angry that Voldemort had left his elf for dead and that's when he turned against Voldemort, because he was so ruthless and he decided to get revenge by destroying the horcrux. Unfortunately he didn't know there were others.

Answer 2

From JKR's webchat of July 30:

"James Farrell: Voldemort never told anyone about his horcruxes, so how on earth did Regulus Black discover his secret?
J.K. Rowling: Horcrux magic was not Voldemort’s own invention; as is established in the story, other wizards had done it, though never gone as far as to make six. Voldemort dropped oblique hints; in his arrogance, he did not believe anybody would be clever enough to understand them. (He does so in the graveyard of Little Hangleton, in front of Harry). He did this before Regulus and Regulus guessed, correctly, what it was that made Voldemort so convinced he could not die"

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