The radical cleric contacted by accused Ft. Hood gunman Maj. Nidal Malik Hasan has such unmistakable connections to past terrorist plots that his e-mail exchanges with the American should have triggered an all-out investigation, a number of officials and experts now believe.
Anwar al Awlaki is an extremist whose sermons have helped radicalize terrorists from Atlanta to New Jersey to London, including cases in which the U.S. military was targeted. A well-spoken Yemeni American, Awlaki has emerged as the leading ideologue for a homegrown generation of young militants who conspire over the Internet.
The fact that Hasan -- the Army psychiatrist accused of killing 13 people at Ft. Hood, Texas, a week ago -- exchanged e-mails with Awlaki in Yemen should have raised grave concern, U.S. officials said.
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.
Lawmakers have called for inquiries on the handling of the case. Rep. Peter Hoekstra of Michigan, the senior Republican on the House Intelligence Committee, said that the contacts between Hasan and the cleric "should have set off red flags regardless of the content."
In response, senior federal law enforcement officials said that they did not have legal justification to pursue a formal investigation because the 10 to 20 e-mail exchanges between Hasan and Awlaki over the last year were not deemed to be violent or threatening.
Awlaki has left a well-documented trail of influence in a string of recent terrorism cases in North America and Europe.
His ideas apparently drove five young men convicted last year of planning a shooting spree against soldiers at Ft. Dix, N.J. In a wiretapped conversation, Eljvir Duka, 23, exulted about an Awlaki sermon that is revered by militants, according to court documents.
"It's a lecture by Imam Anwar . . . brother, I want everyone to hear about it," said Duka, an ethnic Albanian immigrant from Macedonia. "Because he gives it to you raw and uncut. I don't give a damn what everybody says. This is Islam; this is the truth right here."
In cases that also resulted in convictions, London extremists transcribed Awlaki lectures while plotting bombings, and an Atlanta man who filmed potential terrorism targets -- including Washington landmarks and U.S. military bases -- praised the cleric during his trial this summer.
Awlaki's extremist materials also were found in the possession of accused accomplices of the suicide bombers who hit London's transport system in 2005, said Evan Kohlmann, an independent anti-terrorism consultant who served as an expert witness in their trial.
And Canadians now on trial on suspicion of plotting to attack their Parliament received recordings of the cleric's sermons by e-mail from a Briton in Manchester who recruited for Pakistani training camps, according to court documents.
Nonetheless, the FBI has said that investigators decided that Hasan's e-mail exchanges with Awlaki were a legitimate part of the psychiatrist's academic research.
"How is it that the FBI didn't say, even if this looks religious, even if it seems innocuous, something is very wrong?" asked 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?"
A central question, according to experts, is whether authorities decided to pursue their preliminary inquiry into Hasan as a criminal case or a terrorism tip.
Hasan's communications with Awlaki appear to have been intercepted by the National Security Agency, which eavesdrops on e-mails and phone calls around the world, according to a former senior U.S. intelligence official.
But the official said that the investigation may have stalled if the FBI handled it as a criminal matter.
"If you view it as a cop thing, you're caught up in reasonable doubt, probable cause and all those things that don't play on the intel side as strongly," said the former official, who requested anonymity when discussing intelligence matters. "If you play it on the cop side, you're less prone to connect dots because of the higher predicate requirements."
The investigative lapse also may reflect a lack of knowledge of the extremist underworld, combined with a well-meaning reluctance to question religious expression, according to anti-terrorism experts.
"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."
Experts compare Awlaki's stature in the global extremist subculture to the notoriety of two London-based clerics: Abu Qatada and Abu Hamza al Masri.
Qatada, a Jordanian, and Masri, an Egyptian, rose to prominence in the late 1990s, radicalizing scores of militants and helping build Al Qaeda networks across Europe and the Muslim world. Qatada has been jailed pending deportation to Jordan; Hamza was convicted on terrorism charges in 2006.
Unlike the ideologues who instruct young followers to fight only in armed conflicts in Muslim lands, Awlaki appeals to fanatical militants because he preaches all-out global holy war, Kohlmann said.
"Hasan was looking at some of the exact same materials that many homegrown terrorists were looking at," he said. "Awlaki is preaching to a very narrow-minded group of people. A minority of a minority of a minority. . . . He says jihad is where you make it. Even if you are one man, living in a Western nation, it's your obligation to wage jihad."
Investigators said that Hasan and Awlaki did not appear to be close, although Hasan worshiped at the mosque in Falls Church, Va., where Awlaki was an imam in 2001. Awlaki went to London in 2002 and moved to Yemen in 2004.
Hasan appeared to be familiar with Awlaki's political views and religious teachings, which the imam updates frequently on his website, investigators said.
In recent years, Hasan expressed opposition to the wars in Iraq and Afghanistan and spoke out against what he saw as inherent conflicts for U.S. Muslim troops fighting in Muslim nations.
An Awlaki posting from July appears to track closely with Hasan's views. Titled "Fighting Against Government Armies in the Muslim World," it was translated and released Wednesday by Kohlmann.
Awlaki wrote that the armies of the U.S. and its Western allies were "the defenders of apostasy in the Muslim world. They fight against Sharia [Islamic law] and kill the Muslims who attempt to bring it back."
Fighting those armies is therefore justified, Awlaki wrote, according to the translation.
Awlaki, who was born in New Mexico in 1971, has been on the U.S. radar screen for a decade or more.
FBI agents and other counter-terrorism officials have tried to interrogate him since he moved to Yemen. Awlaki was arrested by Yemeni authorities in 2006 and released in late 2007, according to news reports there.
In February, FBI Director Robert S. Mueller III traveled to Yemen to argue for cooperation and access to some suspected Al Qaeda militants and influential supporters such as Awlaki, federal law enforcement officials said.
Greg Miller in the Washington bureau contributed to this report.