After Bigger accidentally kills Mary Dalton, he throws her body into the furnace to burn up.
Mary's remains are discovered when journalists are allowed to investigate the house. While the journalists are standing around in the basement talking, Peggy asks Bigger to clear out some of the ashes from the furnace. He does so, but there are still too many ashes in the furnace, so the basement fills with smoke. This causes one of the journalists to scoop some of the ashes onto the floor. Because of this, the journalists find bits of human bone and Mary's earring in the ashes.
Up until this point, Mary has simply been understood as missing, and Bigger has hoped to exploit the situation to ask for ransom. Now, however, it is clear that Mary has been murdered. Bigger flees and then rapes and kills Bessie. At this point, the authorities have put two and two together. They mount a manhunt for Bigger and finally capture him.
In short, if the journalists had not congregated in the basement and had not seen the bone fragments and earring among the ashes, Bigger might have gotten away with his crime—at least for a longer period of time.
Bigger is driven by fear of whites and has no context for understanding what is happening once he enters white culture. This leads him to react out of terror rather than reason when he kills Mary.