While it is impossible to answer this question directly -- as others point out, we don't know which readings have provided context for your classes -- we can draw from other sources.
For example, the lunatic fringe "9/11 Truther" movement believes that the attacks were predicated by the U.S. Government as a pretext for wars in the Middle East. In this model, the attacks were justified by governmental desire for war.
Doctors Quintan Wiktorowicz and John Kaltner show the explicit message in Al Qaeda's teachings about Jihad and the spread of Islam.
Wikipedia has a reasonably comprehensive article on possible justifications, or at least motives.
David Bukey wrote a good article on the religious motives for terrorism and suicide attacks for The Middle East Quarterly.
And, of course, a Google search will turn up every possible justification you could think of. The trick is finding sources you accept as credible and rejecting subjective views based on bias and ideology.