I remember I was second year college at that time and when I got home from school that was the news in almost all tv stations.  It continually showed plane that hit the World Trade Center.  It is surreal as you see how a building melts like a candle.  Different stories of survival surpassed after that tragic incident. 

I do not know if there were any mystery behind it.  It was merely pure terrorism act.  All I know during that time was it was done by terrorist group Al Qaeda of Afghanistan.  That is all what I know.