Why didn't Dumbledore seek out Horcruxes earlier?

Question

Dumbledore knew for sure that the Dark Lord would return. He said so to Snape - on the night Harry's parents were killed when Snape came to him in anguish.

He also knew about Horcruxes at that time (he said that Voldemort will return).

Yet, he spent seemingly very little effort trying to find out the Horcruxes until the latter years of Harry's schooling.

At the very least, he could have figured out at least that the Diadem was the Ravenclaw's Item Horcrux (didn't take too much imagination) - and being Headmaster, he would have had ample opportunity to question Gray Lady - and heck, he probably could have figured out her and Baron's story from Nearly Headless Nick or the older Headmaster portraits.

Answer 1

Yes, Dumbledore did know that Voldemort would return:

‘Yes, sir. Well, Voldemort’s going to try other ways of coming back, isn’t he? I mean, he hasn’t gone, has he?’

‘No, Harry, he has not. He is still out there somewhere, perhaps looking for another body to share...not being truly alive, he cannot be killed¹. He left Quirrell to die; he shows just as little mercy to his followers as his enemies. Nevertheless, Harry, while you may only have delayed his return to power, it will merely take someone else who is prepared to fight what seems a losing battle next time – and if he is delayed again, and again, why, he may never return to power.’

Philosopher's Stone - page 216 - UK - chapter 17, The Man With Two Faces

Thing is, there are multiple ways of returning a spectral form of a witch or wizard to a corporeal form -- the philosopher's stone; unicorn's blood; possession of another person; the resurrection potions (of Voldemort's creation). The Horcruxes kept Voldemort's spirit alive, but did not prevent him from being greatly debilitated. Dumbledore, however, did not know about the Horcruxes until the time of the events of Chamber of Secrets

As Kevin mentioned in his comment, regarding the ghosts, Seamus Finnigan asks Nearly Headless Nick why the Baron is so bloody:

‘How did he get covered in blood?’ asked Seamus with great interest.

‘I’ve never asked,’ said Nearly Headless Nick delicately.

Philosopher's Stone - page 93 - UK - chapter 7, The Sorting Hat

So it's possible if Nearly Headless Nick didn't know about The Grey Lady and the Bloody Baron, then the other ghosts didn't either. The Grey Lady had a big motivating factor to not tell anyone her story: personal shame over the theft of the diadem from her mother, Rowena Ravenclaw. Canon doesn't specify.

I will say it does seem hard to believe that Dumbledore himself didn't know the story; however, if he had known, I think he would have immediately questioned the Grey Lady and would have undoubtedly found out about the diadem (because he's Dumbledore!) We have to consider the possibility that he hadn't gotten that far in his search for clues and Horcruxes.

Dumbledore did not know about the Horcruxes at the time of James and Lily's death or during the next eleven years. It wasn't until after he viewed Slughorn's memory with the Horcrux conversation between Slughorn and Tom Riddle during Harry's 6th year that Dumbledore knew unequivocally that there were multiple Horcruxes, although he suspected that possibility after the events in the Chamber of Secrets:

‘The careless way in which Voldemort regarded this Horcrux seemed most ominous to me. It suggested that he must have made – or been planning to make – more Horcruxes, so that the loss of his first would not be so detrimental. I did not wish to believe it, but nothing else seemed to make sense.' (Dumbledore)

Half-Blood Prince - page 468-469 - UK - chapter 23, Horcruxes

¹ _{Here is where Dumbledore articulates that he knows Voldemort has not fully died and, more importantly, cannot die. I think this is the first seed that is planted that will ultimately morph into Dumbledore's understanding that Horcruxes are involved.}

Answer 2

Dumbledore did seek and attempt to find about the Horcruxes, for a long time. All the lengths to which he went, such as talking to Bob Ogden, the Ministry Law Enforcement official before he died, talking with the co-founder of Borgin and Burkes, getting the memory from Morphin Gaunt in Azkaban, getting memory from Hokie the house-elf of Hepzibah Smith, etc., all showed that Dumbledore suspected and tried hard to find the whereabouts of the Horcruxes for a long time. He only needed Slughorn's memory to know once-and-for-all that Voldemort created Horcruxes and he wanted to know exactly how many did Voldemort create.

Answer 3

Dumbledore probably DID ask the grey lady if she knew where the diadem was, but may i remind you that the only reason Voldemort was given this information was because he was very persuasive and had a certain way of getting whatever he wanted done, he must've been sweet, a good listener, and very calm with the grey lady. When Harry asked for the diadem he was told where it was because the grey lady had said that he reminded her of a boy (Voldemort) that was very persuasive as well. Dumbledore probably wasn't as convincing which led for the grey lady not to trust him...I'm sure he must've been a good listener and very sweet but not convincing enough to get such information out of her. Another thing is that he probably didn't know the diadem was a horcrux and seeing as how ghosts tend to not know much about other ghosts, he had no reason to go around asking them anything suspicious. Maybe he didn't know that Voldemort tampered with such an item.

Answer 4

He suspected all along that Harry was a horcrux. He knew that Voldemort created them early on, but not where each of them were, or how many were created. He didn't find any of them except the ring. He suspected that Harry was a horcrux as well, and later confirmed this, and knew that Voldemort would need to be returned to power before he could be destroyed for good, since the soul fragment inside Harry could only be destroyed by Voldemort himself.

He then made the mistake of trying to destroy the ring himself was received the death curse that Voldemort had placed on it to prevent tampering. He had been planning on spending the next few years teaching Harry about horcruxes and helping him destroy him, but instead had to improvise a new plan, which involved helping Harry find a horcrux, explain what they were, and leave him the things needed to destroy them, while also being careful not to give too much info in case Voldemort gained access to it through Harry's mind, and leaving Severus to both help Harry, and spy on Voldement, as well as keep the children of Hogwarts save while at the same time seem like he isn't. Basically the most overworked character in the entire series.

Answer 5

I was just reading HBP again, in chapter 23 (Horcruxes) when Harry and Dumbledore are viewing the complete memory, Slughorn says that horcruxes are a banned subject. "Dumbledore's particularly fierce about it." Which seems to indicate that Dumbledore knew all about horcruxes before Tom left Hogwarts.