9/11 Report and US sleeper cells

In light of the atrocity in London, this section of the 9/11 Commission's report is a cause for grave concern:

7.1 FIRST ARRIVALS IN CALIFORNIA
In chapter 5 we described the Southeast Asia travels of Nawaf al Hazmi, Khalid al Mihdhar, and others in January 2000 on the first part of the "planes operation." In that chapter we also described how Mihdhar was spotted in Kuala Lumpur early in January 2000, along with associates who were not identified, and then was lost to sight when the group passed through Bangkok. On January 15, Hazmi and Mihdhar arrived in Los Angeles. They spent about two weeks there before moving on to San Diego.1

Two Weeks in Los Angeles
Why Hazmi and Mihdhar came to California, we do not know for certain. Khalid Sheikh Mohammed (KSM), the organizer of the planes operation, explains that California was a convenient point of entry from Asia and had the added benefit of being far away from the intended target area.2

Hazmi and Mihdhar were ill-prepared for a mission in the United States. Their only qualifications for this plot were their devotion to Usama Bin Ladin, their veteran service, and their ability to get valid U.S. visas. Neither had spent any substantial time in the West, and neither spoke much, if any, English.3

It would therefore be plausible that they or KSM would have tried to identify, in advance, a friendly contact for them in the United States. In detention, KSM denies that al Qaeda had any agents in Southern California. We do not credit this denial.4 We believe it is unlikely that Hazmi and Mihdhar-neither of whom, in contrast to the Hamburg group, had any prior exposure to life in the West-would have come to the United States without arranging to receive assistance from one or more individuals informed in advance of their arrival.5

KSM says that though he told others involved in the conspiracy to stay away from mosques and to avoid establishing personal contacts, he made an exception in this case and instructed Hazmi and Mihdhar to pose as newly arrived Saudi students and seek assistance at local mosques. He counted on their breaking off any such relationships once they moved to the East Coast.6 Our inability to ascertain the activities of Hazmi and Mihdhar during their first two weeks in the United States may reflect al Qaeda tradecraft designed to protect the identity of anyone who may have assisted them during that period.