请分析下面的长句,找出主干并分析附加成分,并翻译。 A three-year independent investigation into the September 11, 2001 attack on the Pentagon has yielded new eyewitness evidence which, according to the Southern California-based researchers who conducted the investigation conclusively (and unfortunately), establishes as a historical fact that the violence which took place in Arlington that day was not the result of a surprise attack by suicide hijackers, but rather a military black operation involving a carefully planned and skillfully executed deception.