<p>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.</p>

Twenty-five years on from the deadliest terror attack in history, BBC investigative journalist Jane McMullen asks why thousands of grieving families are still fighting for answers.  
 
In early 2000, two young Saudis arrived in San Diego. They had never been to America, spoke almost no English, and had no idea how to open a bank account or rent a flat. Within weeks they had done all of it. Twenty months later they hijacked a plane and crashed it into the Pentagon. 
 
McMullen investigates the hidden story of a man who helped two of the hijackers - and reveals a story that the US and Saudi governments have fought to keep secret.