Shelby's company was contracted to upgrade some software for the FBI. Alex had her suspicions that Shelby was, or was as least aiding, the terrorists. As long as Miranda was on Shelby's side, there was no way the upgrade was going to stop. Alex had no choice; she had to let Miranda in on Shelby's secret. During the ensuing interrogation, Shelby secretly let Alex know she was on their side, she just couldn't tell her without overtly without the terrorists finding out. Watch this scene from "Fast," Quantico season 1 episode 19, and don't miss Quantico SUNDAYS 10|9c on ABC.