Editor's Note: (Peter Bergen is CNN's national security analyst, a vice president at New America and a professor of practice at Arizona State University. He is the author of the new book "United States of Jihad: Investigating America's Homegrown Terrorists.")
(CNN) The news that the FBI had investigated the Orlando terrorist, Omar Mateen, in 2013 and 2014 after he made inflammatory comments to coworkers and because he had possible connection to fellow Floridian Moner Abu-Salha -- an al Qaeda recruit who later became the first known American suicide bomber in Syria -- has generated considerable criticism of the bureau. Critics charge that it should have done more to stop Mateen.
The Mateen case is, unfortunately, far from an exception. In a number of the lethal terrorist attacks in the United States since 9/11, the FBI has investigated the perpetrator before the attack, but closed the case because there wasn't sufficient evidence (what the law enforcement community calls "derogatory information") found to continue the investigation.
So is the FBI failing at its most important job? Not necessarily, as the FBI quite properly has to work within guidelines set by the U.S. attorney general about how long an inquiry can remain open in the absence of the kind of derogatory information that would lead to a full-blown investigation.
Indeed, the FBI is often critiqued, quite properly on occasion, not for its laxity but its over-zealousness -- routinely using sting operations to ensnare would-be terrorists, for instance.
The solution is not to give the FBI greater leeway in terrorism investigations, but it is to look for other common threads linking together domestic terrorists who were previously known to the FBI -- namely, the fact that three of the most notorious among them were able to purchase legally and without restraint the kinds of lethal weapons that could maximize the violent impact of their murderous intentions.
Take Carlos Bledsoe, a convert to Islam from Memphis, Tennessee, who killed a U.S soldier at a military recruiting center in Little Rock, Arkansas, in 2009. Bledsoe first attracted the FBI's attention after Yemeni authorities arrested him in November 2008. When he was detained, Bledsoe had in his possession a fake Somali identification card and manuals about how to make bombs and gun silencers. His cell phone contained contacts for militants who were wanted in Saudi Arabia.
Some weeks into Bledsoe's imprisonment, an FBI special agent traveled to Yemen to interrogate him. The FBI had opened an "assessment" of Bledsoe, which, while not a full-scale investigation, showed that he was on its radar. Though Bledsoe insisted that he had gone to Yemen only to teach English and learn Arabic, the FBI agent informed him that he would be deported to the United States.
Following the FBI's instructions to report to them when he was back in Tennessee, Bledsoe met again with an FBI official, who tried to recruit him as an informant. When Bledsoe refused, the FBI stopped tracking him.
This decision is hard to reconcile with the information the FBI had at the time. Bledsoe had been arrested in Yemen with incriminating materials. Perhaps this information was not properly communicated to the FBI at the time by the Yemeni intelligence service, but even so, Bledsoe's arrest should have been enough to raise concern about what he might do next to keep him in their sights.
Michael Leiter, the then-director of the U.S. National Counterterrorism Center, believes that the FBI erred by not sharing their information about Bledsoe with the rest of the intelligence community. "There should have been broad dissemination on this," he later said.
Even Bledsoe expected that the Bureau would follow him after his interviews. To test whether he was under FBI surveillance, Bledsoe bought a semiautomatic rifle over-the-counter at a Wal-Mart. He figured if anyone was watching, they would have stopped him.
A few months after Bledsoe opened fire at the recruiting center in Little Rock -- killing Pvt. William Long and wounding Pvt. Quinton Ezeagwula -- U.S. Army Maj. Nidal Hasan killed 12 soldiers and a civilian at Fort Hood, Texas. Nidal, too, had been a subject of interest at the FBI.
Hasan sent Anwar Awlaki, an American cleric in Yemen who would take on an important leadership position in al Qaeda, a series of emails beginning in December 2008, asking his advice about "Muslims in the US military." In particular, he was interested in the question of whether an American Muslim soldier who turned his gun on fellow U.S. soldiers "in the name of Islam" would be fighting a "true jihad." And if he died in such an assault, would he be considered a shaheed, or a martyr for Islam?
In one lengthy e-mail on May 31, 2009, Hasan suggested that suicide attacks might be "permissible" particularly in cases where the aim was "to kill enemy soldiers," even in instances where there was collateral damage to innocent bystanders.
Taken together, the 18 emails Hasan exchanged with Awlaki reveal a chillingly persistent line of inquiry: Hasan was seeking an Islamic justification for the killing of fellow American soldiers in a suicide mission that might also kill civilians.
Astonishingly, the FBI knew about the entire correspondence. After Hasan sent his first message to Awlaki in 2008, a member of the Joint Terrorism Task Force in the San Diego office of the FBI reviewed it and wrote to colleagues: "Here's another e-mail sent to Aulaqi by a guy who appears to be interested in the military. The header information suggests that his name is 'Nidal Hasan,' but that might not be true. The IP address resolves to Reston, VA. Can we check to see if this guy is a military member?"
After some digging around, the agent in San Diego found a Nidal Hasan assigned to the Walter Reed military hospital in Washington, D.C. Concerned by this discovery, in FBI parlance he "set a lead" to the FBI's Washington Field Office (WFO), which has handled many prominent terrorism cases. The WFO did its own investigation and responded, "Given the context of his military/medical research and the content of his, to date, unanswered messages, WFO does not currently assess Hasan to be involved in terrorist activities."
Agents in the FBI San Diego field office found the response puzzling. The Washington office had examined only Hasan's personnel files, which were completely innocuous. A far more useful approach would have been to talk to some of Hasan's superiors, or even to Hasan himself, which they did not do.
The San Diego agents, confused, emailed back, asking if perhaps Hasan was a "friend," or informant, for the WFO. They wrote: "The response looks a little slim, i.e. limited probing into this individual's background, no contact w/ command and no interview of Hasan. We were wondering if we were missing something, i.e. we need to read between the lines (Hasan is a friend of WFO)?"
The WFO wrote back that Hasan was not "'a friend of WFO.' If you have additional information regarding HASAN's links to terrorism or request any specific action, please share and we will re-assess."
One of the San Diego agents called the FBI office in Washington to explore the issue further and was told, "This is not San Diego, it's DC and we don't go out and interview every Muslim guy who visits extremist websites. Besides, this guy has legitimate work-related reasons to be going to these sites and engaging these extremists in dialogue."
After this, the FBI dropped all investigation of Hasan. It was then and remains now an astonishing choice by the FBI Washington Field Office to dismiss Hasan as a serious threat. After all, a militant Islamist with constant access to U.S. military bases was in a position to do considerable damage. And there was no shortage of red flags in the e-mails themselves.
Hasan subsequently legally purchased the two guns that he used in his November 5, 2009, assault at Fort Hood that left 13 people dead and 32 others injured.
And of course, we all know by now about the bloody effects of Omar Mateen's legally-purchased AR-15.
Omar Mateen, Carlos Bledsoe and Nidal Hasan -- three of the most prominent domestic terrorists since 9/11 -- were FBI subjects of interest. Yet all legally purchased semi-automatic weapons as a prelude to their murderous rampages. In the future, such purchases of weapons must, at a minimum, be flagged to law enforcement or, even better, simply barred altogether.
So as we rightly ask questions about what the FBI knew or should have done to prevent the attack in Orlando, the FBI and other law enforcement entities must also put procedures in place going forward to prevent the subjects of terrorism inquiries from having easy access to the tools to create mass violence.
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