Bamford argues that the NSA in 2000 and 2001 had not only the means, but also the actual information necessary to prevent the attacks on New York and Washington. The agency had been monitoring communications out of an Al Qaeda command center in Yemen, and those had pointed squarely to the presence of two key plotters in California. Yet Hayden at that moment didn’t want to risk any semblance of monitoring people in the United States, even though there was plenty of latitude to tap those two terrorists under the Foreign Intelligence Surveillance Act (FISA).

Decius wrote:

Was this in the 9/11 report?

I couldn't find any mention of a command center in Yemen or communications intelligence about 9/11 plotters in California.

Regarding US persons, Section 3.4 states:

The law requires the NSA to not deliberately collect data on U.S. citizens or on persons in the United States without a warrant based on foreign intelligence requirements. Also, the NSA was supposed to let the FBI know of any indication of crime, espionage, or "terrorist enterprise" so that the FBI could obtain the appropriate warrant. Later in this story, we will learn that while the NSA had the technical capability to report on communications with suspected terrorist facilities in the Middle East, the NSA did not seek FISA Court warrants to collect communications between individuals in the United States and foreign countries, because it believed that this was an FBI role. It also did not want to be viewed as targeting persons in the United States and possibly violating laws that governed NSA's collection of foreign intelligence.

In a footnote, that paragraph is attributed to Michael Hayden interview (Dec. 10, 2003).

Perhaps Bamford drew on working group products (or even a full transcript of the Hayden interview?) that contained relevant details not presented in the final report.

Also, in chapter 6:

After the disruption of the plot in Amman, it had not escaped notice in Washington that Hijazi had lived in California and driven a cab in Boston and that Deek was a naturalized U.S. citizen who, as Berger reminded President Clinton, had been in touch with extremists in the United States as well as abroad. Before Ressam’s arrest, Berger saw no need to raise a public alarm at home -- although the FBI put all field offices on alert. Now, following Ressam’s arrest, the FBI asked for an unprecedented number of special wiretaps. Both Berger and Tenet told us that their impression was that more FISA wiretap requests were processed during the millennium alert than ever before.

So the FBI used FISA in 1999, but NSA was reluctant to apply it in 2000.

noteworthy wrote:
So the FBI used FISA in 1999, but NSA was reluctant to apply it in 2000.

Which begs the question of whether information about these California plotters was turned over the FBI.