The opinion may be important in documenting why the F.B.I. was hesitant last summer to seek court authority to search the computer and other belongings of Zacarias Moussaoui, the only person charged in the Sept. 11 attacks. . . . . .Mr. Moussaoui was arrested in Minnesota last August, and F.B.I. officials have acknowledged that their failure to investigate him more fully was among the mistakes tha