Tragedy struck in Washington, D.C. on Wednesday night, as an American Airlines jet collided with an Army Black Hawk helicopter. That crash resulted in the largest aviation disaster the United States has seen since November 2001.

Police boats combed the banks of the Potomac River on Friday, moving slowly under rainy skies and scanning the shoreline as investigators sought clues into the midair collision that killed 67 people and raised questions about air traffic safety around the nation’s capital.