Victims' families gathered near the site where a plane and copter collided and fell into the Potomac River. 'They are all hurt,' an official said.

This data point is one of several key mysteries investigators are exploring as they seek to explain what caused the nation's worst air disaster in more than a decade, aviation experts said.

CNN By Karina Tsui, Andy Rose, Pete Muntean and Alexandra Skores, CNN Washington, DC (CNN) — Crews working at the site of the deadliest aviation