Je vais te donner quelques indices.
I think that he attack on Twin Towers on September the 11th, 2001 is the worst terrorist attack in the USA history. Over 3 thousand people were killed and hurt.
It destroyed two beautiful buildings which cost a lot of money to do.
And even if we still are not sure who did it, it was a real monster because men, women, children or even families were killed or hurt.
Bonne journée =)