Jag tror att han använde Legilimency .
I Half-Blood Prince , efter att ha visat Harry minnet om Morfin Gaunt i Pensieve, säger Dumbledore:
“Yes, but it took a great deal of skilled Legilimency to coax it out of him,” said Dumbledore, “and why should anybody delve further into Morfin’s mind when he had already confessed to the crime? However, I was able to secure a visit to Morfin in the last weeks of his life, by which time I was attempting to discover as much as I could about Voldemort’s past. I extracted this memory with difficulty. When I saw what it contained, I attempted to use it to secure Morfin’s release from Azkaban. Before the Ministry reached their decision, however, Morfin had died.”
Detta visar att Legilimency kan användas inte bara för att "läsa sinne" (förlåt professor Snape) utan också att extrahera minnen från någon annan, till och med mot sin vilja och lagra dem på ett sådant sätt att de kan senare hämtas och ses i en Pensieve.