It was a sorrowful day. The US government lied lied lied about it being Arab hijackers when all the science shows that there were no hijackers.
Hijackers couldn't melt/vaporize steel, nor could they have planted the nanothermite in the three towers, nor could they have caused the twin towers to GRAVITY COLLAPSE at accelerating speeds, nor could they have caused the free fall of WTC7.
Guess who could have done all those things. Guess who had the MEANS, the MOTIVE, and the OPPORTUNITY!