Investigators plan to push forward on Friday with efforts to retrieve the two aircraft involved in a crash in Washington that killed 67 people and raised questions about air safety in the U.S. capital.

The flight data recorder and cockpit voice recorder, known as the black boxes, may offer clues about what went wrong before the crash.

Sixty-seven people are presumed dead after a passenger plane on approach to Reagan National Airport near Washington, DC, collided Wednesday night with a US Army helicopter midair, sending both aircraft into the Potomac River below,