A history of recent extremist attacks in Europe
Apparently coordinated explosions at the Brussels airport and a metro station killed dozens on Tuesday.
ISIS has claimed responsibility for the attacks, which happened just days after the top suspect in the recent Paris attacks had been arrested in his hometown of Brussels, after which he told officials he had been planning other attacks from there.
The attack in Brussels was just the latest in a string of extremist attacks in Europe since the 2004 Madrid train bombings.
Paris attacks
On Nov. 13, 2015, Islamic State of Iraq and Syria-linked extremists attacked the Bataclan concert hall and other sites across Paris, killing 130 people.
A key suspect in the attack, 26-year-old Salah Abdeslam, was arrested in Brussels on March 18, 2016. Officials discovered him close to his boyhood home in the Molenbeek neighborhood, which is now notorious for fostering European-born extremists. Prosecutors believe that Abdeslam wasn't just in hiding, but planning further attacks. In the middle of the hunt, another Belgian suspect was named, Najim Laachraoui, who may have been the bombmaker.
The attacks took place across several hours at several sites around the French capital. It is believed that all but one of the nine or so attackers in the Paris attacks died the day of the assault.
European security agencies -- particularly in France and Belgium -- missed a series of red-flags which could have led to greater scrutiny of the suspects in the attacks.
Several of the nine suspects had been flagged to French agencies by Turkey or other nations, and the man believed to have planned the attacks, Belgian national Abdelhamid Abaaoud, bragged earlier this year about being able to move freely between Syria and Europe.
After the attacks, Judge Marc Trevidic, who spent 10 years leading counterterrorism investigations for the French national courts, said: "Of course there are security flaws. If there weren't, an organized terrorist attack of that level wouldn't have occurred."
"They are simply overloaded ... we've got security services who have been hiding the fact that they could no longer cope," Trevidic told France Inter, part of France's public radio network.
He said attacks and planned attacks that have been thwarted -- dozens this year alone, according to officials -- have been stopped "by pure luck."
Shootings in Denmark
On Feb. 14, 2015, a gunman killed Danish filmmaker Finn Noergaard and wounded three police officers in Copenhagen. A short time later the gunman, Omar El-Hussein, attacked a synagogue, killing a Jewish guard and wounding two police officers before being shot dead.
El-Hussein was 22 years old and had a background in criminal gangs.
Officials said it was possible he was imitating the assaults from a month earlier in which Islamic radicals carried out a massacre at the Charlie Hebdo newsroom in Paris followed by an attack on Jews at a kosher grocery store.
The first shooting happened on a Saturday afternoon when the gunman used an automatic weapon to shoot through the windows of the Krudttoenden cultural center during a panel discussion on freedom of expression featuring a Swedish artist who had caricatured the Prophet Muhammad. The artist, Lars Vilks, was whisked away unharmed by his bodyguards, but Noergaard was killed and three police officers were wounded.
The attack at the synagogue occurred hours later, shortly before 1 a.m. Sunday.
About four hours later, the shooter was confronted by police as he returned to an address that they were keeping under surveillance.
Charlie Hebdo massacre/kosher grocery attack
On Jan. 7, 2015, a gun assault on the Paris offices of the satirical magazine Charlie Hebdo killed 12 people. Al Qaeda in the Arabian Peninsula claims responsibility for the attack, saying it was in revenge for Charlie Hebdo's depictions of the Prophet Muhammad. A gunman also attacked a Jewish supermarket in Paris, killing four people, and claimed to be inspired by ISIS.
The attack began when masked gunmen stormed the Paris offices of the satirical newspaper. According to witnesses, the armed and masked men walked into the headquarters of the Charlie Hebdo magazine and opened fire in the entrance hallway, killing people as they saw them. The gunmen reportedly sought out members of the newspaper's staff by name during the rampage through the 2nd floor office, which lasted between five and 10 minutes.
The staff was in an editorial meeting and the gunmen headed straight for the paper's editor, Stephane Charbonnier - widely known by his pen name Charb - killing him and his police bodyguard first, said Christophe Crepin, a police union spokesman. Minutes later, two men strolled out to a black car waiting below, calmly firing on a police officer, with one gunman shooting him in the head as he writhed on the ground, according to video and a man who watched in fear from his home across the street.
Brothers Cherif and Said Kouachi were named as the perpetrators. They died in a shootout with police.
In the attack at a Jewish supermarket at Porte de Vincennes, a close friend of the Kouachi brothers, Amedy Coulibaly, shot and took hostages. He said he coordinated his assault with the Charlie Hebdo attackers, but pledged his allegiance to ISIS. He died during the rescuing of his hostages.
Brussels Jewish Museum shootings
On May 24, 2014, in a one-minute rampage that deeply shook Europe's Jewish community, four people were killed at the Jewish Museum in Brussels by an intruder with a Kalashnikov. The accused is a former French fighter linked to ISIS in Syria.
Mehdi Nemmouche was named as the top suspect. The 29-year-old French national had been extradited to Belgium just six months before the attack. He was arrested a few months after the attack by French officials who said they swooped in "minutes" after he set foot on French soil.
Nemmouche is believed to have converted to radical Islam during a stint in a French prison. He was found in possession of firearms, ammunition and a video claiming responsibility for the May 24 attack.
The Lee Rigby murder in London
On May 22, 2013, two al Qaeda-inspired extremists ran down British soldier Lee Rigby with their car in a London street, then almost beheaded him with crude knives in a gruesome assault witnessed by numerous bystanders.
Images of Michael Adebolajo, 29, holding a butcher knife and cleaver with bloodied hands in the moments after the killing shocked people around the world and sparked fears of Islamist terrorism in Britain.
The self-described "soldier of Allah" was given a life sentence for the murder, along with his accomplice, 22-year-old Michael Adebowale, who received a minimum 45-year sentence.
The pair was convicted in in late 2013 of murdering Rigby, a 25-year-old fusilier. Adebowale attacked his torso, while Adebolajo attempted to cut off the soldier's head with a cleaver.
Toulouse attacks
In March of 2012, a gunman claiming links to al Qaeda killed three Jewish schoolchildren, a rabbi and three paratroopers in separate incidents in southern France both in and near Toulouse.
The gunman, Mohammed Merah, had filmed all three killings, and claimed to have posted them online.
Merah was later killed while jumping out of an apartment window during a shootout with police.
The killings began on March 12, when Merah shot a paratrooper. Three days later, using a motorbike, he committed a drive-by shooting on French soldiers waiting by a bank machine.
Finally, on March 19, a rabbi and his two young sons were killed by Merah as they waited for a bus. The extremist then chased down a 7-year-old girl, shooting her dead at point-blank range.
French prosecutors said Merah had been to Afghanistan twice and had trained in the militant stronghold of Waziristan in Pakistan. His brother had been implicated in a network sending fighters to Iraq.
Merah told police he belonged to al Qaeda and wanted to take revenge for Palestinian children killed in the Middle East. He had been under surveillance for years for having "fundamentalist" views.
Norway youth camp massacre
On July 22, 2011, anti-Muslim extremist Anders Behring Breivik planted a bomb in Oslo, then launched a shooting massacre on a youth camp on Norway's Utoya island, killing 77 people, many of them teenagers.
Breivik, now 37, left a long, rambling manifesto about the murders, and during his trial made many eccentric pronouncements about his motivations. Despite all that, he was deemed sane during his trial for the killings, to which he confessed. He was sentenced to life in prison.
Maintaining he acted out of "goodness, not evil" to prevent a wider civil war, Breivik vowed, "I would have done it again."
During the sentencing phase of his trail, Breivik said he wanted to issue an apology, but it wasn't for the victims, most of the teenagers gunned down in one of the worst peacetime shooting massacres in modern history.
"I wish to apologize to all militant nationalists that I wasn't able to execute more," Breivik said.
Frankfurt airport shootings
On March 2, 2011, Islamic extremist Arid Uka shot dead two U.S. airmen and injured two others at Frankfurt airport after apparently being inspired by a fake Internet video purporting to show American atrocities in Afghanistan. It was the first-ever terrorist attack in Germany by an Islamic extremist.
The night before the crime, Uka said in court that he followed a link to a video posted on Facebook that purported to show American soldiers raping a teenage Muslim girl. It turned out to be a scene from the 2007 anti-war Brian De Palma film "Redacted," taken out of context.
He said he then decided he should do anything possible to prevent more American soldiers from going to Afghanistan.
Six months later, he said he now does not understand why he went through with the killings.
"If you ask me why I did this I can only say ... I don't understand anymore how I went that far."
Uka went to the airport armed with a pistol, extra ammunition and two knives. Inside Terminal 2, he spotted two U.S. servicemen who had just arrived and followed them to their U.S. Air Force bus. After several servicemen boarded the bus, he approached some near the rear, asked for a cigarette, boarded the bus, and then started shooting.
7/7 London bombings
On July 7, 2005, 52 commuters were killed in London when four al Qaeda-inspired suicide bombers blew themselves up on three subway trains and a bus during rush hour.
Four British men were ultimately blamed for the attack. Police claim they just barely foiled a second similar attack mere weeks later.
At least two of the suicide bombers spent time at an al Qaeda camp in Pakistan to prepare themselves for the attack.
The coordinated assault was the first suicide attack in Western Europe as well as the deadliest attack in London since World War II. News that the four young attackers were all born or raised in Britain stunned many and strained ties between the country's large Muslim community and the wider population.
Madrid train bombings
On March 11, 2004, bombs on four Madrid commuter trains in the morning rush hour killed 191 people and wounded another 1,400.
The main suspect, who was arrested in Italy, was identified as Rabei Osman Ahmed, a 33-year-old Egyptian. Many of those arrested for the attack were picked up in Belgium or Italy.
Osman Ahmed was identified by people living near a decrepit rural cottage where the bombs used in the attack were assembled. Fingerprints of several key suspects were found in the cottage.
The attack was determined to have been carried out with a string of 10 backpack bombs that struck morning rush-hour commuter trains.
The militants who took credit for the bombings said they were acting on behalf of al Qaeda to avenge the presence of Spanish troops in Iraq and Afghanistan. Spanish investigators said, however, that the cell did not receive orders or financing from Osama bin Laden's terrorist group, but was inspired by it. Twenty-nine people were put on trial in Madrid over the attacks.
Shortly after the bombings, Spain withdrew its troops from Iraq and Afghanistan.