They find out that Dan killed Keith in season 4, episode 18 ("The Runaway Found") of One Tree Hill. In this episode, Abby Brown reveals to Lucas that she witnessed Dan murder his brother Keith through the blinds of a classroom. This revelation is a major turning point in the series, as Lucas learns the truth about Keith's death from Abby's confession, leading eventually to Dan's turn to the police in the season 4 finale.
Related
when do the phillies play next
just now
1
when do new episodes of gen v
just now
0
when do they find sophia in the walking dead
just now
0