Via Michael Isikoff in Newsweek:

In the aftermath of the 9/11 attacks, the Justice Department secretly gave the green light for the U.S. military to attack apartment buildings and office complexes inside the United States, deploy high-tech surveillance against U.S. citizens and potentially suspend First Amendment freedom-of-the-press rights in order to combat the terror threat...

The October 23, 2001 memo (from John Yoo to Alberto Gonzales and William Haynes)also said the 4th Amendment could be disregarded:

At another point, the memo advices: "Military action might encompass making arrests, seizing documents or other property, searching persons or places or keeping them under surveillance, intercepting electronic or wireless communications, setting up roadblocks, interviewing witnesses or searching for suspects."

Here's the memo (pdf.)