As the lead agency on two anti-terrorism task forces that reviewed the e-mails, the FBI should have pursued a “full field” investigation, said one counter-terrorism official who spoke Wednesday on condition of anonymity because the case was ongoing. That would have allowed agents to interview Hasan’s friends and colleagues and take other steps that might have detected suspicious behavior.

“What possible conversations could you have with Awlaki that could not be red flags?” the official said, adding that the e-mails should have triggered a more aggressive investigation by law enforcement, intelligence and military officials…

“How is it that the FBI didn’t say, even if this looks religious, even if it seems innocuous, something is very wrong?” said Kohlmann, who also served as an expert witness in the Ft. Dix and Atlanta cases and in the trial of the Briton linked to the Canadian case. “This person is not a mainstream cleric. Why was Hasan focusing on him? Why did he view him as a legitimate source of information?”…

“It seems that the American investigators had difficulties detecting signs of worrisome conduct,” Jean-Louis Bruguiere, a veteran French anti-terrorism judge, said in a telephone interview. “It may also be that, because of the respect for religion, and the excesses by the U.S. services in recent years, that today there’s a tendency to be too prudent — perhaps less vigilant.”