WASHINGTON - The final report of the commission investigating the Sept. 11, 2001, attacks will not conclude definitively that the terrorist strike could have been prevented, but it is clear that U.S. officials missed a series of opportunities to stop some of the hijackers, disrupt the plot and perhaps save lives. The most significant signal appears to have come in April 2000,when Niaz Khan, a Briton of Pakistani heritage, walked into the FBI’s office in Newark, N.J. Khan told agents that he had been trained by al-Qaida, the terrorist network that carried out the Sept. 11 attacks, and that there...