The National Security Agency four months ago intercepted conversations among leaders of Al Qaeda in Yemen discussing a plot to use a Nigerian man for a coming terrorist attack, but American spy agencies later failed to combine the intercepts with other information that might have disrupted last week’s attempted airline bombing. The electronic intercepts were translated and disseminated across classified computer networks, government officials said on Wednesday, but analysts at the National Counterterrorism Center in Washington did not synthesize the eavesdropping intelligence with information gathered in November when the father of the would-be bomber, Umar Farouk Abdulmutallab, visited the United...