I noticed this passage in the One Percent Doctrine by Ron Suskind (pp. 93-94). It is further evidence that the NSA intercepted some of the hijackers' calls to/from the US before 9/11:

“FBI investigators had been interviewing [FBI agent] Coleman and others throughout the winter, seeking context on several key NSA dispatches that had been discovered in the days after 9/11. Most notable among them were calls NSA had collected in 2000 from San Diego to a number in Yemen. The Yemen number was for the daughter of a man who, Coleman told investigators, “was the uncle of half the violent jihadists we knew of in the country.” This was the number—so familiar to Coleman from his work prosecuting al Qaeda that he knew it by heart—the 9/11 hijacker Khalid al-Mihdhar had called while he hid out in San Diego. In fact, Coleman and other FBI al Qaeda specialists had even placed an order with the NSA back in 1998—that any calls between the Yemen line and the US be passed to the bureau—that the NSA didn't fill. “For us,” Coleman said, “anyone who called the Yemen number is white-hot, a top suspect.”

Hattip: PT

I have some comments:
(1) If you hadn't already heard, the NSA intercepted some of the hijackers' calls.