There is a push after 9/11 to justify domestic spying and to overturn the efforts of the civil rights movement in protecting U.S. citizens from abusive government.
A troubling aspect about this debate is that the 9/11 tragedy was executed by non-U.S. citizens. So even if you do concede that spying on U.S. citizens may be justified under certain circumstances, how would such actions preventing 9/11? 9/11 is being exploited by government agencies to push agendas (that pre-date 9/11) to increase their power over U.S. citizens and to weaken Constitutional protections.
Websites Looking into: http://www.epic.org/privacy/terrorism/usapatriot/