what the US is shy of?

This is a screen-shot from FBI website. As claimed by the US, It is well known that Osama-Bin-Laden was the primes suspect/accused behind the demolition of WTC Towers.(9/11). I was shocked to see that FBI has primarily mentioned the Crime as "MURDER OF U.S. NATIONALS OUTSIDE THE UNITED STATES" . Though they have accpeted the fact by mentioning "ATTACK ON A FEDERAL FACILITY RESULTING IN DEATH"?? I dont understand why the US is feeling shy of mentioning the crime in black and white? Or Osama is not behind the attack on 9/11?