A hijacked airliner, with 58 people reported on board, was heading toward the Pentagon, an FBI official told AFP.
"[Ronald Reagan] National Airport reported it about a minute out by radar," the official told AFP.
Shortly after the warning, a fighter jet overflew the Pentagon building which had already been hit by two explosions, leaving at least seven dead and many other wounded.
Police had cleared the area around the building and set up an emergency makeshift medical unit under a highway overpass to treat the wounded.
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At some distance from the Pentagon, crowds of people could be seen watching the dramatic scene unfold below.
AFP