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.

Sixty passengers and four crew members from the plane and three Black Hawk helicopter personnel were killed, and a recovery mission is underway.

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.