at the same time, hundreds more fighters were returning to europe without specific terrorist missions, and hundreds of europeans had been radicalized at home without visiting syria. any one of them could have posed a threat. >> you know, 5,000 european nationals that have gone out to syria and iraq, we don't know exactly how many have come back, we kind of figure around a third of that, so, you know, 1,500 or more. so it's extremely difficult, i think, to get it right in terms of who do we monitor and how do we monitor them. because we don't have the resources or indeed the culture in our society to put 24/7 surveillance on thousands of citizens every day. >> narrator: to make matters worse, the 28 countries of the european union are often wary about sharing intelligence with each other. >> (translated): the protection

of sources and methods makes it difficult to share intelligence. when you see the difficulty we have in europe coming to agreement on basic subjects, it's even harder when it comes to intelligence sharing. >> narrator: as more and more isis recruits returned to europe, the authorities in france and belgium were >> (translated): every week people would come back from syria, there was nothing except syria; it was all about syria. there were so many cases related to syria that people who should have been watched just could not be. cleary we can't do everything, and we didn't have the means-- we still don't-- to monitor all of this. >> (translated): that brings up the issue of having to make choices. that is, among all the possibilities of targets to monitor, thousands of targets to monitor, it's necessary to make choices.

>> narrator: in june 2014, french spy chiefs made a fateful decision. for the last three years, they had been monitoring al qaeda veterans of the buttes-chaumont gang: cherif kouachi, and his brother said. meanwhile, kouachi's old associate amedy coulibaly had just been released from prison. french domestic intelligence now decided to stop watching them and shift surveillance resources onto the growing threat from isis. >> (translated): for more than three years the surveillance and wiretappings had yielded nothing, so it was stopped at that time. this was mid-2014, which means that for six months, the services didn't know what the kouachi brothers were up to, and didn't know about the alliance they developed with amedy coulibaly, who had never been monitored since he stepped out of prison.

(man shouting) (gunfire) >> breaking news out of france. the french police right now e hunting for masked gunmen who stormed the offices of the satirical nepaper, opened fire in the french capital today. at least 12 people are dead, four others are in critical condition. >> (translated): charlie hebdo was a turning point. the concept of attacking in the heart of europe was a new expression of terrorism. we realized that it was now a reality. >> narrator: 12 people were shot dead in tcharlie hebdo attacks.

within hours, the names of the killers surfaced. for the former counterterror chief louis caprioli, it was unsettling news. >> (translated): it quickly became known it was the kouachis. that rang a bell immediately. i thought, "yes, these are the people from 2005." so immediately i thought that there might have been a failure somewhere. when an attack happens and you had no prior intelligence, that's one thing. but when you find out that you could have prevented it, that is a tragedy. >> narrator: after a massive manhunt, the kouachi brothers were killed in a shootout with french police. (gunfire) they declared allegiance to al qaeda shortly before they died. that same day, their friend

amedy coulibaly carried out his part of the plot, shooting four people dead in a jewish supermarket before being killed by a swat team. although he had been radicalized by al qaeda, he claimed allegiance to isis. >> (translated): i was surprised. at first i couldn't understand this alliance between coulibaly from isis. and kouachi of al qaeda. how was the association between al qaeda and isis even possible? i realized it's simply a question of relationships. it's just that they knew each other. personal connections sometimes go beyond a group's strategy.

they may fight each other in syria, but can still do things together in france. >> narrator: as france reeled from the charlie hebdo attacks, another plot was being uncovered across the border in belgium. (shouting, gunfire) on the 15th of january, belgian police raided a house in the town of verviers, killing two terrorist suspects and wounding one. u.s. and french spy agencies had helped track their return from syria. investigators found evidence that they were part of an isis cell deployed by the young belgian extremist abdelhamid abaaoud. >> because of the verviers case, abaaoud came very clearly in the picture. >> narrator: eric van der sypt is a belgian counterterror prosecutor. >> we realized that he was active in the recruitment of people from france, from belgium and he trained them, and he was

responsible for sending back people to western europe also. people that want to die, they're not afraid to die, and they're not afraid to kill other people while doing so. >> narrator: abaaoud now became the subject of an international manhunt. european and u.s. intelligence detected his cell phone in athens, but by the time greek police raided his safe house, he had already fled. it was the first of what would be several missed opportunities to capture him. >> abaaoud disappeared, and it's a pity we lost him because we knew who he was, what he was doing, and sure, we would have loved to have captured him, it's a sure thing, but he got away. he managed to escape, and he managed to go back to syria. >> narrator: over the next few months, isis sent a series of le

operatives to attack europe authorities suspected abaaoud was involved. then in june 2015, spanish counterterror officials made a breakthrough. with the help of u.s. intelligence, they detected an alleged isis fighter, who had just returned to europe from syria via poland. the poles arrested and questioned him, along with spanish investigators. his name was abdeljail ait el kaid. >> (translated): a character like el kaid being arrested generated huge expectation from our colleagues in other countries. the whole world wanted to know what el kaid was going to tell us. >> narrator: ait el kaid admitted he'd been sent to europe to commit an attack. >> (translated): and the one organizing it all was abaaoud. he was the brains and the organizer of this brigade, whose

mission was to come back to europe and commit attacks here on european soil. well, from my point of view, we had prevented an attack. we've had someone in front of us who was with abaaoud, who was trained by him, who was chosen by him, and now he was in prison. and well, that's very satisfying. i can't say it any other way. >> narrator: ait el kaid gave up the name of another suspected operative sent by abaaoud to hit european targets. in august, the man was arrested by french police when he returned from syria. his name was reda hame. marc trévidic, who was weeks away from the end of his term as a counterterror magistrate - now questioned hame.

>> (translated): to begin with, he said he had a terrorist mission. the general idea was to shoot a crowd, to shoot people, and his mission was to do so during a rock concert. but the target was only to be given to him later on. there was the idea that, "you go back to france, and we will contact you with encrypted messages. that's how we will operate." he surprised me especially when he explained how much abaaoud wanted to hurt us. he was very clear, he will do whatever it takes, he wanted to commit a huge attack at all costs. it was quite chilling.

>> narrator: on august 21, just one week later, the warnings were confirmed. a heavily armed man, ayoub el khazzani, opened fire on a high-speed train between amsterdam and paris. the gunman was overpowered by three american tourists, and no one was killed. a year earlier, spanish authorities had actually warned their french counterparts about khazzani, who they'd tracked from spain to france. but investigators believe he eventually made his way to syria, where he was allegedly trained and sent back to europe by abaaoud. >> (translated): i had the feeling that these were warning signs, that in fact they were just gaining ground, carrying out smaller operations while they were organizing something bigger. because any terrorist attack occupies all of our investigators, as there is so much work after an attack.

all of this was intended to conceal the bigger plot. >> narrator: abaaoud was now wanted across the continent. >> (translated): but we had no idea where abaaoud could be. apparently no one knew where he was. according to reda hame, he was in raqqa, but we had no way of locating him. >> narrator: in fact, by september, abaaoud was already back in europe laying the groundwork for the most ambitious plot yet. investigators say he slipped in through greece with the help of smugglers. policing of europe's external borders is left largely to individual nations, whose budgets and capabilities vary. >> (translated): so there's a gap, an abyss, and all the terrorists can rush in and easily circulate between europe, france, turkey, and of course syria and iraq.

i believe it's one of the flaws of the european union: it has never been able to ensure the protection of our borders. >> (translated): it is completely porous. we are in a totally open system. they can use real papers, real fake papers, fake fake papers, real real papers. it's horrible, but that's how it is. >> narrator: abaaoud then took advantage of europe's open internal borders to travel freely from country to country. >> (translated): there are just no borders. there are none, clearly they do not exist. although we are under a state of emergency. there are none. we are not an island, we are on the european continent and the enemy is at our doorstep. he can come from either side-- by sea, air, and land. that is the reality.

>> narrator: as the war in syria intensified in 2015, germany declared it would welcome all refugees fleeing the conflict. hundreds of thousands of refugees and migrants flooded into europe, but there was no comprehensive system to vet them. most came in through greece, which was overwhelmed, and it could only thoroughly screen about a third of the arrivals. counterterror chiefs say they warned about terrorists exploiting this opportunity. >> (translated): this is the nightmare; this is the problem. isis claims to have infiltrated many hundreds of fighters-- that's taken from their propaganda. >> narrator: jean-louis bruguière was a top french counter-terrorism judge for more than two decades. he says the migrant crisis exposed the dysfunction of the european union's approach to security. >> (translated): there was no coordinated european policy. germany was playing alone.

france did not agree; no other country agreed on this. so on important, strategic subjects that affect the whole of europe, europe's security, we have difficulty in finding a consensus. >> to be fair, at the time, the sort of mood within germany, the mood around many countries in europe was one of, we're living through a great humanitarian tragedy. these desperately poor afflicted people needed a place of refuge, and it was europe's job to provide that. now we're saying, you know, there's been some dreadful terrorist attacks and on reflection maybe that wasn't the right thing to do. well, you know, that's a little harsh. >> narrator: abaaoud now used the chaos of the refugee flow to get a team of militants into and around europe. two of the team entered europe with refugees on the island of leros. one used a stolen syrian passport flagged by interpol as possibly being used by terrorists. but greek authorities weren't checking interpol's database

regularly, and the men were let through. bomb-maker najim laachraoui and one of the leaders of the cell, mohamed belkaid, are also thought to have entered europe through greece. >> they both had false belgian identity papers. it's a tool they used. i think they used the fact that there were a lot of refugees at that time over there so they could blend in, to stay anonymous. (men shouting) >> narrator: on september 9, laachraoui and belkaid were met at the budapest train station, then packed with refugees, by another suspected member of the cell: salah abdeslam. hours later, police stopped their mercedes at the austria-hungary border. they were known extremists. one was wanted on a terrorism

warrant. one was on an eu watch list. but the police didn't spot anything suspicious during questioning or in their databases, and the car was waved on. >> rotella: why were they let through without further investigation? >> it's very easy after the event to say, "well, we should have got this guy because he was on the record." we're dealing with, you know, 20-plus countries in europe sharing a different set of information systems, not all of them sort of interconnected, not all of them holding sensitive terrorist data. not all the intelligence is shared with all of the partners on all of the systems at the same time, and we have a challenge, i think, in europe where we have different information systems in different parts, in different countries, but also in different parts of the eu architecture that are not hooked up. >> narrator: by the end of october, abaaoud had everything in place: weapons, explosives, targets and the men to hit them.

>> (translated): the question everyone had was not if something would happen, but rather when and where. (gunfire) >> we're coming on the air to tell you about a situation unfolding right now in paris, where there have been a number of apparent attacks. >> ...are dead in multiple attacks across the french capital. there were at least six shootings in various locations. and at this moment, police are storming a concert hall. >> (translated): you can imagine how i felt after all the years i had spent in the anti-terrorist section as i got home on the 13th of november and heard about the bataclan.

i immediately thought of reda hame and the rock concert, and at that moment, you think, (bleep) >> i remember saying to myself, "i hope there is not a belgian connection." and i was proven wrong the following hours already, and the belgian connection, if i may call it like that, was soon very, very clear. >> (translated): we started to understand that the belgians are involved, and suddenly abaaoud's name resurfaces. abaaoud, whom we'd been looking for for months, and who we believed was still in syria. this was obviously a shock. but we also felt uneasy. how did we not see this coming? our intelligence was maybe not as good as it should have been. >> narrator: 130 people were killed in the attacks. most of the suspected plotters

were already known to the authorities, and multiple opportunities to stop them had been missed. at least six were wanted on international arrest warrants for terrorism. one was under police surveillance with wiretaps and a hidden camera. at least seven were on terrorist watch lists. at least 12 were stopped, questioned, and even arrested as they traveled around europe and back and forth to syria to prepare the paris plot. >> these individuals were on the radar. they had traveled to syria. they were known to law enforcement intelligence officials. even with that information in the hands of intelligence and law enforcement, they were able to really carry out large-scale, spectacular, catastrophic-style attacks. because law enforcement and border patrol officials from one country simply don't communicate with their counterparts in another country in a way that would make

information that they possess actionable and really disrupt or stop a terrorist from moving across their border. >> narrator: the paris attacks were staged almost entirely from belgium. that's where the bombs were made, where coordinators directed the attacks by phone. french security chiefs say the belgians should have done more to stop the plot. >> (translated): it's not by chance that the whole network was based in belgium and not in france or italy. they chose a place that is both the weakest link, and where there were networks that existed for years. this group should have been detected. the signs were not small. they had a definite red flag. the belgium authorities did not detect the threat to paris.

this is indisputably an example of a system which has completely failed. >> (translated): it's a bit too easy for all these big countries with a lot of police officers and foreign intelligence services to criticize belgium. we're a small country. we don't have a police culture, we don't have our own foreign intelligence service. it's impossible to keep an eye on everyone. 24-hour surveillance with just two people happens only in tv shows. so this means we can only watch a few individuals. people don't quite realize this. >> narrator: five days after the attacks, abaaoud was tracked down to an apartment on the outskirts of paris. (gunfire) in the battle that followed, one of the plotters detonated a suicide vest. (explosion)

abaaoud's remains were identified two days later. but then investigators discovered that other suspected leaders of the cell were still on the loose. >> (translated): we realized that there were people in belgium who seemed more important than abaaoud. through phone intercepts, we discovered that abaaoud asked for instructions or help from people in belgium. so we got to work on them. >> narrator: seven remaining suspects were holed up in safe houses back in their old neighborhoods in brussels, where they were working on a new plot. unknown to belgian intelligence and the nsa, which was helping hunt the fugitives, the bomb maker, najim laachraoui, was in direct contact with a shadowy isis chief in syria known as abu ahmad.

abu ahmad avoided interception by using encrypted communicatios to give detailed orders and bomb-making instructions... >> the individuals were sharing information, they were getting instruction on how to make explosives from individuals in syria. the content of those communications were encrypted. there's no technological way to intercept those communications. we have not solved this problem. this is a problem that is with us today. >> narrator: investigators say abu ahmad worked closely with another senior isis operate in syria known as abu sulaiyman al fransi. u.s. counterterror officials believe he is a 26-year-old moroccan immigrant who grew up in france, served in the french foreign legion, and did prison time for drug dealing before joining isis. he is now suspected of playing a lead role in overseeing the paris and brussels plots. >> it shows a level of direction from isis, you know, that this is not simply an attack that was inspired by isis propaganda or

online communication; this is an attack that was actually being directed at a degree of specificity by isis central, isis leadership. >> narrator: in march, after a four-month hunt, belgian police finally discovered a series of apartments rented using false identities and finally closed in on the cell. they shot dead one of the suspects and captured another: salah abdeslam. but the bomb maker and others were hiding elsewhere. >> (translated): i thought we were reaching the end point and that there were only a few left. i have to admit that we had no idea there was still an operational unit in belgium.

>> breaking news right now: to explosions rocking the main terminal at brussels airport. there are reports of another attack, an explosion at a subway, a metro station. >> we should underscore that this appears to be a coordinated-type attack. >> narrator: 32 people died in the brussels bombings. isis has vowed more attacks in europe even as they lose ground in syria and iraq. >> (translated): now we realize the extent of the phenomenon, that it's no longer one, two, three dozen individuals, but there are thousands of people in syria and iraq who are ready to die, and that there are thousands here in europe who are also ready to die. the human and technical means in our possession are not proportionate. we won't be able to control, monitor, and wiretap everyone, so at one point, it's evident that people will be able to attack us. >> narrator: in response,

european leaders have set up a new counterterrorism center and recently approved the passenger name record to bolster border security across the continent. but the threat has worsened a political crisis for the europen union. in june, the united kingdom voted in a referendum to leave the eu. other countries are considering doing the same. the counterterror chiefs say the systemic problems remain and europe is as vulnerable as ever. >> (translated): november 13th should never have happened. the brussels attacks should never have happened. the flaws in the european system are multiple. they're institutional. i'm not sure what we're waiting for. do we have to wait for hundreds more deaths? >> (translated): the situation has never been worse, but i

think it's an illusion to believe we'll be able to protect ourselves and live in a sort of bubble. everyone dreams of a society with zero risks, but the zero- risk option does not exist. >> go to pbs.org/frontline for more of our partner propublica's reporting on terror in europe. >> why were they let through without further investigation? >> they have not developed a way to effectively stop somebody from traveling... >> and explore frontline's reporting on confronting the isis threat. visit our watch page, where you can stream more than 200 frontline documentaries. connect to the frontline community on facebook and twitter. then sign up for our newsletter at pbs.org/frontline. >> feels so good to get these off. >> released from prison. >> freedom at last. >> some people think being on parole is you're free. >> i'm better off sometimes in jail. >> frontline examines how one state is trying to change the way parole works. >> the parole system needs to be

completely reimagined. >> and whether it's making a difference. >> sending them out and expecting them to be perfect, to abide by standards that don't apply to the rest of us... >> my life is pretty much ruined for the next (bleep) three years. >> frontlinis made possible by contributions to your pbs station from viewers like you. thank you. and by the corporation for public broadcasting. major support for frontliis provided by the john d. and catherine t. macarthur foundation, committed to building a more just, verdant, and peaceful world. more information is available at macfound.org. additional support is provided by ford foundation, working with visionaries on the front lines of social change worldwide, at fordfoundation.org. the park foundation, dedicated to heightening public awareness of critical issues. the john and helen glessner family trust, supporting trustworthy journalism that informs and inspires. and by the frontline journalism fund, with major support from

jon and jo ann hagler, and additional support from joseph azrack and abigail congdon. >> for more on this and other frontline programs, visit our website at pbs.org/frontline. frontline's "terror in europe" is available on dvd. to order, visit shoppbs.org, or call 1-800-play-pbs. frontline is also available for download on itunes.

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