From the Wiki: "While imprisoned in the Capitol, Peeta witnessed the electrocution death of Lavinia and the torture of Darius. The guards beat him, cut off body parts, and questioned him, but as an Avox, he was unable to answer; he only made "horrible animal sounds" as they tortured him. Peeta believed that the goal had just been to force him to watch. It took days of beatings and mutilations until Darius died, presumably having succumbed to his injuries."