I assume you are asking about the World Trade Center attack on Sept. 11, 2001. There is still some confusion as to who was actually behind these attacks. The identities of some of the reported perpetrators who died in the attack, were found to belong to living people. There are many other inconsistencies with the events of 9/11, including reports of explosives being detonated within the towers, the lack of plane wreckage at the Pentagon and the field in Pennsylvania, etc.
If the official story is to be believed, then it is possible that the terrorists were Muslims. That does not mean that their actions were Islamic in nature, just as the actions of Timothy McVeigh are not an example of Christianity. In fact, Islam forbids these very actions no matter the religion of the perpetrator. A "real Muslim" is one who strives to live Islamically in every aspect of his or her life. So, according to the official story, some of the terrorists were said to be drinkers, smokers, fornicators, etc.; all things that are forbidden in Islam. Islam is the religion and Muslims are judged in accordance to their adherence, not the other way around.