Terry Strada joins a lawsuit on behalf of 9/11 families, while Special Agent Danny Gonzalez and the FBI continue their investigation in secret.
A BBC News Long Form Audio production.
Host: Jane McMullen
Series producer: Neal Razzell
Additional production: Sally Abrahams, Keiligh Baker, Nik Sindle
Editor: Matt Willis
Music: Thomas Ross-Fitzsimons
Sound design: Charlie Brandon-King
Production management: Phoebe Lomas
Radio 4 commissioning executive: Tracy Williams
Radio 4 commissioning editor: Dan Clarke