NNA - U.S. authorities said on Thursday it was not yet clear why a regional jet and a U.S. Army helicopter collided at a Washington airport, killing 67 people in the deadliest U.S. air disaster in more than 20 years.

Investigators recovered the so-called black boxes from the plane, an American Airlines Bombardier jet carrying 60 passengers and four crew members, which collided with an Army Black Hawk helicopter and crashed into the Potomac River as it prepared to land at Ronald Reagan Washington National Airport on Wednesday night.--Reuters 

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