WHENEVER A TERRORIST ATTACK OCCURS , it never takes long for politicians to begin calling for more surveillance powers. The horrendous attacks in Paris last week, which left more than 120 people dead, are no exception to this rule. In recent days, officials in the United Kingdom and the United States have been among those arguing that more surveillance of Internet communications is necessary to prevent further atrocities. The case for expanded surveillance of communications, however, is complicated by an analysis of recent terrorist attacks. The Intercept has reviewed 10 high-profile jihadi attacks carried out in Western countries between 2013 and 2015 (see below), and in each case some or all of the perpetrators were already known to the authorities before they executed their plot. In other words, most of the terrorists involved were not ghost operatives who sprang from nowhere to commit their crimes; they were already viewed as a potential threat, yet were not subjected to sufficient scrutiny by authorities under existing counterterrorism powers. Some of those involved in last week’s Paris massacre, for instance, were already known to authorities; at least three of the men appear to have been flagged at different times as having been radicalized, but warning signs were ignored. In the aftermath of a terrorist atrocity, government officials often seem to talk about surveillance as if it were some sort of panacea, a silver bullet. But what they always fail to explain is how, even with mass surveillance systems already in place in countries like France, the United States, and the United Kingdom, attacks still happen. In reality, it is only possible to watch some of the people some of the time, not all of the people all of the time. Even if you had every single person in the world under constant electronic surveillance, you would still need a human being to analyze the data and assess any threats in a timely fashion. And human resources are limited and fallible. There is no doubt that we live in a dangerous world and that intelligence agencies and the police have a difficult job to do, particularly in the current geopolitical environment. They know about hundreds or thousands of individuals who sympathize with terrorist groups, any one of whom may be plotting an attack, yet they do not appear to have the means to monitor each of these people closely over sustained periods of time. If any lesson can be learned from studying the perpetrators of recent attacks, it is that there needs to be a greater investment in conducting targeted surveillance of known terror suspects and a move away from the constant knee-jerk expansion of dragnet surveillance, which has simply not proven itself to be effective, regardless of the debate about whether it is legal or ethical in the first place.

Map of 10 recent attacks carried out in Western countries by Islamic extremists. freevectormaps.com

1. Paris attacks: November 13, 2015 Victims: 129 dead. 400+ wounded.

Named suspected perpetrators: Ismaël Omar Mostefaï (29; French), Samy Amimour (28; French), Ibrahim Abdeslam (31; French), Bilal Hadfi (20; French), Abdelhamid Abaaoud (27; Belgian), Salah Abdeslam (26; French).

Weapons: Assault rifles, hand grenades, suicide vests. Known to authorities? At least three of the men involved in planning and carrying out the French attacks were known to European authorities and at least four were listed in a U.S. terrorism watchlist database. Ismaël Omar Mostefaï, who helped carry out the massacre at the Bataclan concert venue, had been flagged as a radicalization risk in 2010. French police reportedly ignored two warnings about Mostefaï before he carried out the attacks. Some of his friends claimed to have tried to alert French police about his radical views, but said they were told the authorities could do nothing. Samy Amimour, another of the men involved in the Bataclan massacre, had been previously charged with terrorist offenses “after an abortive attempt to travel to Yemen,” according to Paris prosecutors. The alleged ringleader of the attacks, Abdelhamid Abaaoud, was also well-known to European police. In 2013, he booked a flight from Cologne to Turkey, which was flagged to German authorities because he was reportedly on an EU watchlist. But he was not detained and was able to board the flight. From Turkey, Abaaoud entered Syria, where he joined ISIS. Abaaoud later returned to Europe and was named as a wanted extremist in January following a gun battle in Belgium. In February, he featured prominently in ISIS propaganda magazine Dabiq boasting about how he had been able to evade police detection in Europe. Others involved in the Paris attacks are also likely to have been on the radar of police and intelligence agencies due to their travels to Syria. Bilal Hadfi, for instance, was living in Belgium after having returned from Syria, where he is believed to have fought with Islamic State militants. Hadfi apparently attended the Instituut Anneessens-Funck college in Brussels; his former history professor recalled that, following the Charlie Hebdo massacre in January 2015, Hadfi defended the attacks. The professor reported him to management due to concerns about his radical views, but management “decided not to intervene, to avoid stigmatizing the young student.” In June, Hadfi reportedly posted on his Facebook page encouraging terrorist attacks: “Those dogs are attacking our civilians everywhere. Strike them in their community of pigs so they can’t feel safe again in their own dreams.” The family of Ibrahim Abdeslam, who detonated a suicide vest inside a cafe during the attacks, said he too had spent “a long time” in Syria before returning to Europe.

Photo: Jacques Brinon/AP

2. Thalys train attack, France: August 21, 2015 Victims: No deaths. Two wounded.

Alleged perpetrator: Ayoub El Khazzani (26; Moroccan).

Weapons: Pistol, assault rifle, box cutter, bottle of petrol. Known to authorities? Khazzani was reportedly known to European authorities for his Islamic radicalism. While living in Spain, he had come to security agencies’ attention after he was observed defending jihadis and attending a radical mosque in Algeciras, Spain.

French police stand guard on the platform next to a Thalys train at the station in Arras, northern France, on Aug. 22, 2015, the day after an armed gunman was overpowered by passengers. Photo: Philippe Huguen/AFP/Getty Images

3. Curtis Culwell Center attack, Garland, Texas: May 3, 2015 Victims: One wounded.

Perpetrators: Elton Simpson (30; American) and Nadir Soofi (34; Pakistani-American).

Weapons: Assault rifles, handguns. Known to authorities? Elton Simpson had reportedly been placed on the U.S. no-fly list and had been convicted of a terror-related offense in 2011 after being caught discussing traveling to Somalia to engage in violent jihad. Soofi, on the other hand, was reportedly “relatively unknown to federal investigators,” though he lived with Simpson. A third man, Abdul Malik Abdul Kareem, was allegedly responsible for supplying the guns and ammunition used in the attack. Kareem was investigated in 2012 after he was suspected of developing a plot to attack a Super Bowl game in Arizona with explosives.

An FBI evidence response team member investigates the crime scene after a shooting outside of the Curtis Culwell Center in Garland, Texas, May 4, 2015 . Photo: Ben Torres/Getty Images

4. Shootings in Copenhagen, Denmark: February 14-15, 2015 Victims: Two dead. Five wounded.

Perpetrator: Omar Abdel Hamid El-Hussein (22; Danish-Jordanian-Palestinian).

Weapons: Assault rifle, pistols. Known to authorities? Hussein was reportedly well-known to Danish security agencies. Prior to the Copenhagen shootings, he had been imprisoned for stabbing a teenager in the leg on a train. While he was in jail, prison officials filed a concern report to the Danish intelligence agency PET, warning that his behavior had changed and that he had become extremely religious. Two weeks after he was released from jail he went on the shooting rampage that left three dead and five wounded in different parts of Copenhagen. Shortly before the attacks, Hussein had apparently sworn allegiance to ISIS in a post on his Facebook page.

The body of a shooting suspect lies on the pavement as Danish forensic police officers examine the scene, Feb. 15, 2015, Copenhagen Denmark. Photo: TV2 Norway/AP

5. Shootings in Paris (Charlie Hebdo and Jewish supermarket): January 7-9, 2015 Victims: 17 dead. 20 wounded.

Perpetrators: Chérif Kouachi (32; French), Saïd Kouachi (34; French), Amedy Coulibaly (32; French).

Weapons: Assault rifles, submachine guns, grenade launcher, pistols, shotgun. Known to authorities? Chérif Kouachi was well-known to French security agencies as an Islamic extremist. In 2005 he was detained trying to board a plane for Syria and in 2008 he was jailed for three years for his role in sending militants to Iraq. Both Chérif and his brother Saïd were alleged to have been involved in a 2010 plot to free from prison Smaïn Ait Ali Belkacem, the French-Algerian extremist responsible for the 1995 Paris metro station bombing. The brothers were never prosecuted over the prison-break plot due to a lack of evidence. In 2011, Saïd traveled to Yemen and allegedly trained with al Qaeda. The U.S. reportedly provided France with intelligence in 2011 showing the brothers received terrorist training in Yemen and French authorities monitored them until the spring of 2014. Amedy Coulibaly was also well-known to the authorities. In 2013 he was sentenced to five years in prison for providing ammunition as part of the 2010 prison-break plot that the Kouachi brothers were also suspected of involvement in. However, Coulibaly reportedly only spent about three months in jail and was released in March 2014.

Screen grab of the attack on Charlie Hebdo magazine, Paris, France, Jan. 7, 2015. Photo: Youtube

6. Cafe seige, Sydney, Australia: December 15-16, 2014 Victims: Two dead. Four wounded.

Perpetrator: Man Haron Monis (50; Iranian-Australian).

Weapon: Shotgun. Known to authorities? Two months prior to taking 17 people hostage in a Sydney cafe, Monis wrote a letter to Australia’s attorney general seeking advice about the legality of communicating with ISIS. He was “well-known” to federal and state police, as well as the Australian Security Intelligence Organization, and had sent “hate letters” to families of Australian soldiers killed in overseas conflicts. Before carrying out his attacks, Monis apparently pledged allegiance on his website to Islamic State leader Abu Bakr al-Baghdadi. This was reported to Australian authorities, who reviewed Monis’ website and social media posts but (erroneously) concluded he was unlikely to carry out an act of violence.

A hostage runs toward tactical response police officers after escaping from a cafe under siege in the central business district of Sydney, Australia, Dec. 15, 2014. Photo: Bob Griffith/AP

7. Canada attacks (Quebec car ramming and parliament shooting): October 20 and 22, 2014 Victims: Two dead. Four injured.

Perpetrators: Michael Zehaf-Bibeau (32; Canadian-Libyan) and Martin Couture-Rouleau (25; Canadian).

Weapons: Rifle, car. Known to authorities? Couture-Rouleau was known to Canadian authorities prior to an attack in which he rammed two Canadian soldiers, killing one and injuring another. He had reportedly been “considered some kind of threat by the Canadian government” and had posted a variety of pro-jihadi materials on his Facebook page. Police had been monitoring him over concerns that he had become radicalized and his passport had been seized to prevent him from traveling abroad to join militants. Zehaf-Bibeau, who shot dead a soldier at a war memorial near the Canadian parliament, was a habitual offender who had a criminal record for a number of offenses, including robbery and drug possession. Zehaf-Bibeau was reportedly “on the radar” of federal authorities in Canada and his email address had been previously found on the computer hard drive of someone charged with a “terrorist-related offense.”

Police, bystanders and soldiers aid a fallen soldier at the National War Memorial in Ottawa, Canada, the site of an apparent terrorist attack, Oct. 22, 2014. Photo: Cuddington/Barcroft Media /Landov

8. Jewish Museum killings in Brussels: May 24, 2014 Victims: Four dead.

Perpetrator: Mehdi Nemmouche (29; French)

Weapons: Automatic rifle, handgun. Known to authorities? Nemmouche had been incarcerated on five occasions in France for various crimes, including armed robbery. In 2013 he had traveled to Syria. When he returned to Europe he was reportedly placed under surveillance by French counterterrorism police, who suspected he had joined with Islamic extremist fighters while in Syria.

Photo: Yves Logghe/AP

9. Beheading in Woolwich, London: May 22, 2013 Victims: One dead.

Perpetrators: Michael Adebolajo (28; British-Nigerian) and Michael Adebowale (22; British-Nigerian).

Weapons: Cleaver, knives, pistol. Known to authorities? Both attackers were known to British authorities and were suspected of having been radicalized prior to their murder of soldier Lee Rigby in Woolwich, London. According to a U.K. parliamentary report published following the attack, Adebolajo was investigated under five separate police and security service operations. He was believed to have links to several extremist networks and was suspected of having tried to travel overseas to join a terrorist organization. Adebowale was investigated by British spies after he was identified as having viewed extremist material online. London counterterrorism police also received an uncorroborated tip that Adebowale was affiliated with al Qaeda. Investigators reviewed Adebowale’s cellphone records and apparently did not find anything of interest. But they did not check his landline call records, which if they had would have revealed that he had been in contact with an individual in Yemen linked to al Qaeda. Covert surveillance of both Adebolajo and Adebowale had ceased prior to their attack in London in May 2013, though Adebowale was still the subject of a terrorism-related investigation at the time.

A British police officer carries an evidence bag containing a knife near the scene where a soldier was murdered in Woolwich, Britain, May 23, 2013. Photo: Facundo Arrizabalaga/EPA /Landov

10. Boston Marathon bombing: April 15, 2013 Victims: Five dead. 260+ wounded.

Perpetrators: Dzhokhar Tsarnaev (19; Kyrgyzstani-American) and Tamerlan Tsarnaev (26; Kyrgyzstani-American).

Weapons: Pressure-cooker bombs, semi-automatic pistol, improvised explosive devices. Known to authorities? Dzhokhar’s older brother, Tamerlan, who orchestrated the attacks, was placed on two different U.S. government watchlists in late 2011. Russian security agency FSB tipped off the FBI and CIA in 2011 that Tamerlan “was a follower of radical Islam,” and he and his family were subsequently interviewed by American agents, according to the Associated Press. The CIA reportedly “cleared [Tamerlan] of any ties to violent extremism” two years before he and his younger brother carried out the bombing of the marathon.