If you look at the 19 hijackers who came to the United States in Sept. 11 to commit those acts, if you'd looked at them before they got onto a plane, you could probably say the same thing. There were various levels of expertise, various levels of competence. There were various roles that each of them played. In this particular group in Lackawanna, you could say that this is a similar, potentially effective, terrorist group.

Our information is that they never did anything that whole summer you were watching them.

You could consider them a terrorist cell. They've certainly been to the training camp. They certainly were poised to commit an act. Consequently, I think it's fair to call them a terrorist cell.

It's important to us, because it is one of those cases [in] which we've been able to take the information from the other intelligence agencies, such as CIA, work together with the intelligence information that we developed in the United States, and put it together to disrupt a group of individuals that potentially were in the position to commit a terrorist attack against the United States. So it is a good example of what we're able to do now with the benefit of the Patriot Act, with the benefit of the additional resources to address terrorism within the United States.

Robert Mueller was sworn in as director of the FBI on Sept. 4, 2001, one week before the terrorist attacks of Sept. 11. He tells FRONTLINE that the Patriot Act was an important tool in the Lackawanna investigation because it allowed U.S. intelligence agencies to break down barriers to information sharing between intelligence and criminal investigations in order to disrupt a "terrorist cell." Mueller also describes the reasons why this information sharing is necessary and responds to critics who say that it could increase the potential for civil liberties abuses. This is an edited transcript of an interview conducted on Sept. 12, 2003.