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 "manhunt: the ten-year search for bin laden -- from 9/11 to abbottabad." david sterman is a program associate at new america, a washington-based think tank.









(cnn) on thursday morning, four u.s. marines were killed in chattanooga, tennessee, when a gunman shot at two separate military facilities: a military recruiting center and a navy training reserve center.









the suspected shooter is 24-year-old mohammad youssuf abdulazeez, who also is dead, according to the fbi.









the shooter's motivations are as yet unclear. u.s. attorney bill killian told reporters that the investigation is being treated as "act of domestic terrorism."









one likely reason why investigators are treating the shooting as a potential domestic terrorist attack is that there are multiple cases of jihadist extremists plotting to attack military facilities and recruiting centers in the united states.









military facilities and personnel are a common target in jihadist plots to conduct violence within the united states. nearly a third of the 119 americans accused of plotting an attack inside the united states since 9/11 were alleged to have plotted to attack u.s. military targets, according to data collected by new america









thursday's shooting would not be the first jihadist attack on a u.s. military recruitment office, nor even the first one with a connection to tennessee.









on november 18, 2008, bledsoe was arrested in yemen for possessing a fake somali identification card. the fake identity card was part of bledsoe's ill-conceived plan to travel to somalia to wage jihad. when he was arrested, bledsoe was found to possess manuals about how to make bombs and gun silencers. on his cell phone were contacts for militants who were wanted in saudi arabia.









the fbi interviewed bledsoe after his arrest in yemen, and he eventually returned to the united states. in a letter to the judge in his case, bledsoe claimed to have been sent by al qaeda in the arabian peninsula (aqap) and portrayed the little rock shooting as a jihadist attack -- though there is no evidence that he was actually sent or directed by aqap.









the fort hood attack helps illustrate the particular role of military targets for jihadists who see themselves as engaged in a war with the united states and soldiers as legitimate targets.









hasan's only real confidant in texas was duane reasoner jr., an 18-year-old covert from catholicism who attended his mosque. hasan told reasoner he didn't want to be deployed to afghanistan. at their final dinner together, on november 4, hasan told reasoner that what he really wanted was to quit the military because anyone fighting against fellow muslims was likely to go to hell.









the next day, the 467th combat stress control detachment to which hasan was assigned was due to report at the soldier readiness processing center at fort hood -- the last stop before the unit shipped out to afghanistan. this was the day that hasan selected to conduct his deadly attack.









earlier this year, an alleged plot inspired by isis to attack a military base was foiled in illinois. on march 25, hasan edmonds , a 22-year-old u.s. citizen, and his cousin jonas edmonds, a 29-year-old u.s. citizen, were arrested. the two allegedly plotted for hasan, a member of the illinois national guard, to travel to syria to fight with isis while jonas would carry out an attack on a military facility. the two were monitored by an undercover officer.









another potential reason to consider jihadist terrorism as a motivation in the chattanooga shooting is that the incident comes amid a spike in terrorism cases this year, driven in large part by the threat posed by individuals inspired by isis and a law enforcement crackdown on potential plotters.









already less than seven months into 2015, more americans have been charged in jihadist terrorism related cases than in any other year since 9/11, according to data collected by new america









moreover, the timing of the chattanooga shooting on the final night of ramadan raises another flag. isis called for its supporters to unleash "a month of disaster" during the holy celebration of ramadan, which ends friday.









regardless of the shooter's motivation, it is essential to investigate all possibilities and not jump to conclusions.









on september 16, 2013, aaron alexis killed 12 people in a shooting at the washington navy yard. though the attack came on the heels of the anniversary of 9/11, occurred in the nation's capital, targeted a military facility and had other characteristics that superficially suggested it might be jihadist terrorism, alexis turned out to not be a terrorist, but a mass shooter with a history of mental health problems.









even though alexis was not a jihadist terrorist, his shooting at the navy yard demonstrated in the words of a department of the navy report on the shooting that there were "critical performance gaps" in the navy yard's capabilities "against a wide range of threats" and that "the naval support activity washington's antiterrorism program" was "deficient in several areas."









an issue laid bare by thursday's shootings is the challenge of securing military recruiting offices from attacks.









while u.s. military bases tend to have high levels of security, military recruiting offices do not. since these offices have been the scenes of two attacks in the past six years, the pentagon should consider how to make them harder targets.
chattanooga shooting: history of attacks on u.s. military
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