This is the quote I had in mind, from Chapter 23 of HBP:

‘He [Voldemort] seems to have reserved the process of making Horcruxes for particularly significant deaths. You would certainly have been that. He believed that in killing you, he was destroying the danger the prophecy had outlined. He believed he was making himself invincible. I am sure that he was intending to make his final Horcrux with your death.

‘As we know, he failed. After an interval of some years, however, he used Nagini to kill an old Muggle man, and it might then have occurred to him to turn her into his last Horcrux.

If Dumbledore is right, then Bertha Jorkins could not have been murdered to make that Horcrux, because she was already dead. Is there an interview where Rowling says otherwise? I don't see anything on the wiki page (a citation, or other reference) that backs up their claim.