Episode 4x02 "Are you there God? It's Me, Dean Winchester"
MEG AND AGENT HENRICKSEN RETURN TO HAUNT SAM AND DEAN Sam (Jared Padalecki) and Dean (Jenson Ackles) are stunned when the spirits of Meg (guest star Nikki Aycox) and Agent Henricksen (guest star C. Malik Whitfield) appear and accuse the Winchesters of failing them. Although rattled, the brothers must pull together to save Bobby (Jim Beaver), who is too crippled with guilt to fight off the angry ghosts of the young children he couldn't save. Phil Sgriccia directed the episode. Written by Sera Gamble
Source: Futon Critic