In this chapter of the Anushasana Parva of the Mahabharata, different sages tell Yudhishthira about the benefits they received by praying to Shiva. In particular, the sage Valmiki, author of the Ramayana, says this:
Once upon a time, in course of a dialectical disputation, certain ascetics that were possessors of the homa fire denounced me as one guilty of Brahmanicide. As soon as they had denounced me as such, the sin of Brahmanicide, O Bharata, possessed me. I then, for cleansing myself, sought the protection of the sinless Isana who is irresistible in energy. I become cleansed of all my sins. That dispeller of all sorrows, viz., the destroyer of the triple city of the Asuras, said unto me,--Thy fame shall be great in the world
My question is, what is this story of Valmiki being accused of killing a Brahmin? And did he actually commit the crime? Valmiki was originally the son of Varuna the ocean god, but he was raised by robbers and became a robber himself, so is it possible that he killed some Brahmana along the way?
And if he was innocent, did some "ascetics that were possessors of the homa fire" falsely accuse him just to win a debate against him? What was this debate about? Are there any other scriptures which describe this story?