Crews are on the scene on the Potomac River to retrieve the submerged wreckage of an airliner and an Army helicopter.

Just offshore of the towering national monuments of Washington, a tall crane now hovers over the Potomac River. It casts a large shadow over the effort to understand what caused the collision of a passenger jet and a military helicopter last Wednesday as a new phase of the recovery starts.

Salvage crews have recovered an engine and large pieces of fuselage and are working to retrieve a wing from the wreckage of a commercial airliner involved in last week’s midair collision near Washington’s Reagan National Airport.