I'm sure that you are right to think that Dumbledore would have been a member of the Wizengamot, or had at least had been following famous trials. Of course that was before the 1945 defeat of Grindelwald, so Dumbledore might not have been elected yet to the Wizengamot. It still doesn't mean he could not have noted who gave what testimony. Morfin and Marvolo were tried for practising magic on a muggle and for assaulting Bob Ogden in the performance of his duty. And I am also sure that Dumbledore was aware that Morfin was rearrested and given a life sentence at Azkaban because of the Riddle killings.

I think it was the coincidence of Morfin being charged for the Riddle killings, having a sixth year student at school called Tom Marvolo Riddle, who coincidentally had taken to openly wearing a curious ring, and extracting the memory from Morfin of what happened that night. Don't forget that until the end of his days Morfin bemoaned the loss of the Peverell Ring. And that also Dumbledore, once he recovered that memory, did try to get Morfin released. Perhaps in trying to get to the bottom of Morfin's memories, Dumbledore found it useful to contact Bob Ogden as well as Caractacus Burke and Hokey the House-Elf, also incarcerated in Azkaban. But don't forget also that Dumbledore had been looking for the Resurrection stone for a long time, and maybe suspected the Gaunts could have had it. Maybe he even saw Marvolo wearing the ring at his trial.

Especially as Dumbledore clearly remembered talking to Mrs Cole and her recollections of Tom's behaviour at the orphanage. And Dumbledore also could remember how he set alight Tom's cupboard, and the revelation of the stolen thimble, mouth organ, yoyo etc. The no doubt sensational murder of Hepzibah Smith, allegedly by her House Elf, Hokey, also may have revived his hunt for more memories of Tom Riddle. Hokey's memory would have led Dumbledore back to interviewing Caractacus Burke, who could confirm Hepzibah's reference to the ragged, heavily pregnant girl who sold the locket.

And that in itself is strange. Ever since COS Dumbledore knew about the Diarycrux. And LV's GOF rebirthing provided other clues. Dumbledore clearly knew about the Peverell Ring, or at least the Resurrection Stone, and I agree he may well have seen Tom Riddle wearing the Peverell Ring at school. And we know that Slughorn definitely did see Riddle wearing it at a Slug club meeting. Why else would Dumbledore flash it around like an engagement ring when he and Harry visited Slughorn to persuade him to become Potions master?

That is my view, that it was reading Kreacher's mind, which started Dumbledore off on a search at the Gaunt domicile. If looking for Horcruxes that would be a logical place to start, wouldn't it? Dumbledore would need to check that the Locketcrux was not there, but he found the Ring instead. Then, perhaps, Dumbledore was reminded of the cave.

What I know, I found out after he had left Hogwarts, after much painstaking effort, after tracing those few who could be tricked into speaking, after searching old records and questioning Muggle and wizard witnesses alike.

I contrived my theory of the Wizengamot simply because I didn't know where else Dumbledore could have gotten his information; or how he knew who to contact; in this case Bob Ogden. Your theory about Dumbledore following interesting court cases is certainly a good possibility. We also know from Dumbledore that he follows the Muggle newspapers as well, re: Frank Bryce. He very well might have read the Riddle family orbituaries.....I agree that Dumbledore must have suspected the Gaunts might possess Peverell's ring and as you suggest, Dumbledore very well may have seen Marvolo himself wearing the ring. If Tom could find out who he was; or to what parentage he belonged, all he had to do was look up Wizarding Family records; since Tom knows that his grandfather's name was Marvolo.....

I also agree that Dumbledore at some point spoke with Burke... it is probably safe to say that Burke himself gave Dumbledore information about Hepzibah; and from there Dumbledore could have found out by other means about Hokey; say again from Hokey's own evidence to the Ministry and Wizengamot. Two family heirlooms missing, would have been something Dumbledore was privy to. And as you yourself point out, Hepzibah's murder would be quite a sensational story for Dumbledore not to follow.

Not only that, Dumbledore told Harry that he suspected Voldemort in the creation of a Horcrux for quite some time; however; we are never told how long this period of suspicion was. As a reader, I always assumed that Dumbledore began to suspect that Tom was doing whatever he could to be immortal as far back as Tom's interview for the DADA position. I don't believe Dumbledore suspected Tom as far as Horcruxes go before this time; the memories he extracted were more for what information Dumbledore could gleam about Tom's past given the suspicious events at school. During Voldemort's interview, we are told of his startling physical transformation; Dumbledore might have started to put two and two together then. As you point out, it isn't until COS that Dumbledore found out about the diary; and it isn't until COS that Dumbledore started to believe that Voldemort very possibly had made not one but multiple Horcruxes.

Dumbledore very well might have known even before reading Kreacher's mind where the Gaunt domicile was re: Bob Ogden's memories. So, just because he didn't find the ring until right before he went to fetch Harry from 4PD; doesn't necessarily mean Dumbledore didn't know where the Gaunts lived. Having said this; I go back to the original question: if Dumbledore knew this piece of information for so long, what kept him from going after the ring sooner; what kept him from visiting the Gaunt home in the first place? As a reader, I was under the impression that Dumbledore began the actual Horcrux hunt right around the end of OOTP. First of all, time wise, I don't think it was really possible for Dumbledore to have gone to look for the Horcruxes sooner; or to even go the to the Gaunt house. Dumbledore's primary concern was Harry; Harry is protected while he is at Hogwarts; and to me at least, it was imperative that Dumbledore be present at Hogwarts as well, at least during Harry's school year. Even before Harry became a student at Hogwarts, Dumbledore's activities were primarily concerned with the Order; and fighting Voldemort during the first war. I really can't see Dumbledore actually finding the time to go and look for anything.

In HBP, Dumbledore tells Harry that he began delving into Tom's past after he left Hogwarts; so the circumstantial evidence so to speak: Myrtle's death, the Riddle deaths, Tom wearing the ring at school; other "nasty" incidents at Hogwarts that couldn't be traced to Tom or his followers; inspire Dumbledore to research Tom's past:

The newspapers would have to be a good bet. But I still think Dumbledore might also have wanted to take a closer look at that ring because of his search for the Resurrection stone, rather than because of his suspicions about Tom Riddle's involvement with the Riddle murders. Maybe Bob Ogden might have been one of the few people who was openly prepared to tell Dumbledore about his memories. For we know that Dumbledore had supporters and friends among the clan of Ogden. Tiberius Ogden resigned in protest from the Wizengamot when Dumbledore was expelled from it. Also, Dumbledore's brother would have certainly had bottles of Ogden's firewhiskey at the Hogs Head. We know that even before that interview or Tom Riddle left school, Dumbledore had begun to suspect something of Tom Riddle because he banned the Horcrux book from the library. Also, from his own memory DD knew that Mrs Cole had obeyed Merope's wishes in how her newborn son was to be named, and that Tom Marvolo Riddle knew DD knew. One reason why he feared DD so much.

Again the newspapers would be a good bet, as well as any information that Burke could supply. In those pre-Barty Crouch days Morfin would probably have got a trial, even though as a reoffender who believed himself to be guilty it might have been a mere formality. But I very much doubt that poor Hokey would have been so honoured. It depends if Dumbledore had enough 'pull' to retrieve any evidence she might give to the Department of control over Magical Creatures.

On the other hand, since Cornelius Fudge mentioned that Harry could have got the names of the DE's at LV's rebirthing from old trial records, I can only assume that Hogwarts library was the main repository for old Ministry records. In that case, Dumbledore may not have needed to go far to do research. And you are right about tracing Tom Riddle's parentage through old Wizarding Family records. The library might even have its own copy of Nature's Nobility, which Hermione used to such effect during DH.

I think that Dumbledore did suspect that Tom Riddle made at least one Horcrux whilst at school, probably from Moaning Myrtle's death, or was at least considering it. DD probably hoped that it stopped there, once the book was removed from the library, only to be dismayed by subsequent murders. He probably didn't know what sort of object this Horcrux might be, since a diary is a fairly ordinary object, like the mouth organ we saw at the orphanage. We do know that by the DADA interview DD knew that Tom Riddle was after Founders' objects because of Hepzibah's death, and that he must have been up to something as horrendous as making other Horcruxes because of the noticeable change in LV's appearance. You could almost work out the sequence of Horcrux-making from these changes alone. DD might even have suspected Tom to be after Gryffindor's sword at the DADA interview. So that makes at least two items that DD suspected were Horcruxes . And that is even before DD knew that Harry, himself, probably was a Horcrux.

He had already warned Snape to watch out for Quirrell, who had returned from a journey to Europe, and, during COS, had also cottoned on to who might be opening the Chamber of Secrets but wasn't sure how it could be accomplished. As he told Harry at the end of COS DD knew from his spies that LV had once again retreated to Albania.

There was a window of opportunity after COS, but maybe the escape of Sirius Black stopped DD from doing any exploring, and Frank Bryce was killed soon afterwards. Perhaps that is why DD couldn't do much before the end of OotP. It doesn't explain what DD could and didn't do once he saw that curse-scar on Harry's forehead, let alone before Harry's arrival at Hogwarts.

I'm sure that is why DD suspected Tom Riddle was into Horcrux-making and to start making connections. DD certainly had some idea that Tom Riddle wasn't dead, not only because Harry survived the AK, but also by Quirrell's return and the attempts to steal the Philosopher's Stone.

I think you are right, though. The memory, if nothing else, would have suggested that Slughorn knew quite a bit more than he was letting on about Tom Riddle and horcrux. And I think Dumbledore flashing around that ring certainly put the wind up Slughorn. Perhaps both had guessed it was a horcrux. And because he was fool enough to show the ring off might have persuaded Tom Riddle to hide the ring back at the Gaunt house as well as hide the rest.