More than 700 planes had already taken off and landed at Reagan National Airport on Wednesday when American Airlines flight 5342 approached it through one of the nation’s most congested air corridors.

The collision of a U.S. Army helicopter and an American Airlines plane over Washington D.C. has left many people stunned.

Authorities say it is not yet clear why a jet crashed into an Army helicopter, killing 67 people in the deadliest US air ...