In addition to Adamant's Answer. There is a section in Short Stories from Hogwarts of Power, Politics and Pesky Poltergeists which details why <PERSON> had suspicions about what Lord <PERSON> had achieved during the first wizarding war.
<PERSON> already had suspicions as there was a rumour he had made himself invincible. Then after the defeat of Lord <PERSON>, his suspicions were raised even move given Professor <PERSON> guilt over what he has discussed with <PERSON> at school about Horcruxes. When <PERSON> tries to question him over this <PERSON> retired from the school.
When the wizarding world fell into war, and rumours swirled that <PERSON> had, somehow, made himself immortal, <PERSON> was sure that it was he who had made <PERSON> invincible, by teaching him about Horcruxes (this guilt was misplaced, as <PERSON> already knew how to make a Horcrux, and had feigned innocence in order to find out what might happen if a wizard made more than one). <PERSON> became ill with guilt and fright. <PERSON>, now Headmaster, treated his colleague with particular kindness at this time, which had the paradoxical effect of increasing <PERSON>’s guilt, reinforcing his determination never to tell a living soul what a dreadful mistake he had made.
Lord <PERSON> made no attempt to seize Hogwarts on his first ascent to power. 773
<PERSON> believed, correctly, that he was safest remaining in his post rather than risking the outside world while <PERSON> was at large. When <PERSON> met his match upon attacking the infant <PERSON>, <PERSON> was even more jubilant than most of the wizarding population. If <PERSON> had been killed, <PERSON> reasoned, then he could not have made a Horcrux, which meant that he, <PERSON>, was innocent after all. It was <PERSON>’s extremity of relief, and the disjointed phrases he let fall in the first rush of emotion after hearing of <PERSON>’s defeat, that first alerted <PERSON> to the possibility that <PERSON> had shared Dark secrets with <PERSON>. <PERSON>’s gentle attempts to question <PERSON>, however, caused him to clam up. A few days later, <PERSON> (who had now completed a half century of service to the school) tendered his resignation. 417