Langdon is an Assistant United States Attorney.
Peter, Jesse's brother, confronts him in his office, saying he has the wrong man. Langdon shows photos of Jesse's meetings with Ibrahim and suggests Jesse hit bottom and asked the wrong people for help. Peter doesn't believe him, but Langdon insists surveillance doesn't lie.
Peter later goes to Langdon's office but discovers he took a job in Washington and was replaced with a new AUSA, Janet Wirth, whom he meets with instead.