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.

Crews are on the scene on the Potomac River to retrieve the submerged wreckage of an airliner and an Army helicopter that collided midair in the deadliest U.S. air disaster since 2001.

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