Answer:
The Islamic Terrorist group Al-Qaeda.
Step-by-step explanation:
The terrorist attacks of September 11, 2001 are a well-known series of events that claimed the lives of almost 3000 people and injured 6000 others. These attacks were presumably perpetrated by the terrorist group Al-Qaeda.
There are several conspirative theories that accuse the U.S. government for planning the attacks, but the official veredict remains against Al-Qaeda alone.