The coordinated suicide bombings that killed more than 30 people and wounded 250 more at an international airport and downtown subway station in Brussels on Tuesday were “shocking but not surprising” and shared many of the hallmarks of previous European terror attacks, according to Stanford terrorism experts.

“My research shows that in general, terrorist plots in Europe involve larger numbers of conspirators than do plots in the United States,” said Martha Crenshaw, a senior fellow at Stanford’s Center for International Security and Cooperation (CISAC).

Belgian authorities said that as many as five people may have been directly involved in the bombings, including two Belgian-born brothers with violent criminal records, and that several suspects were linked to the same terrorist network that carried out the deadly Paris attacks last November.

“It is common for terrorist conspiracies anywhere to be formed from prior social groupings – friends and relatives,” said Crenshaw.

“The bonds that link individuals are not entirely ideological by any means. Criminal backgrounds are also not surprising. Indeed prison radicalization is a well-known phenomenon.”

A Notorious Neighborhood

Many of the suspects in the Brussels bombings had ties to the inner-city neighborhood of Molenbeek, a majority Muslim enclave of mostly Moroccan descent with a long history as a logistical base for jihadists.

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“Brussels and particularly Molenbeek is one of those places that comes up a lot when you’re talking about counter terrorism,” said Terrence Peterson, a postdoctoral fellow at CISAC.

“You do have terrorism networks that use these areas, in the same way that organized crime does, to thrive…It seems to be the place where all the networks are locating in part because Belgian security hasn’t been very effective in fighting terrorism.”

Foreign Fighters Bring the War Home

Belgium is a small nation, with a population of around 11 million people, but it has the highest per capita percentage of any Western country of foreign fighters who have joined the battle in Iraq and Syria, according to a recent report, which estimated the total number at 440.

“People were even saying it was not a matter of if, but when Belgium was attacked,” said Joe Felter, a CISAC senior research scholar and former Colonel in the U.S. Army Special Forces.

“You’ve got a high concentration of radicalized individuals in that neighborhood of Brussels, so logistically it was easier for them to recruit, plan and coordinate the execution of these attacks. Local residents loading up explosive packed suitcases in a cab and driving across town to the airport exposes them to much less risk of compromise than would a plot requiring cross border preparation and movement by foreign citizens.”

Felter said he was concerned that the Brussels bombings, for which the Islamic State group has claimed responsibility, would inspire copycat attacks in other countries.

“The real risk now is these home-grown, self-directed terrorist attacks,” he said.

“A successful attack like this, with all its media attention and publicity, is only going to inspire and motivate more attempts going forward.”

Former U.S. Secretary of State and presidential candidate Hillary Clinton said European nations needed to do a better job of sharing intelligence to track foreign fighters as they returned home, during a foreign policy speech at Stanford’s Freeman Spogli Institute for International Studies on Wednesday.

“The most urgent task is stopping the flow of foreign fighters to and from the Middle East,” Clinton said.

“Thousands of young recruits have flocked to Syria from France, Germany, Belgium and the United Kingdom. Their European passports make it easier for them to cross borders and eventually return home, radicalized and battle-hardened. We need to know the identities of every fighter who makes that trip and start revoking their passports and visas.”

Turkey’s president announced at a press conference on Wednesday that his country had deported one of the suspected Brussels bombers back to the Netherlands last year with a clear warning that he was a jihadi.

Identifying Hot Spots

Clinton said authorities also needed to work to improve social conditions in problem areas such as Molenbeek.

“There…has to be a special emphasis on identifying and investing in the hot spots, the specific neighborhoods, prisons and schools where recruitment happens in clusters as we’ve seen in Brussels,” Clinton said.

Other European countries such as Denmark, which has also been struggling to deal with a high percentage of foreign fighters, are trying to proactively to discourage citizens from travelling to Syria to fight, said Anja Dalgaard-Nielsen, former executive director of the Danish Security and Intelligence Service and a CISAC affiliate.

“Politicians are likely to talk about tougher legislation, but there are also measured voices, calling for a strong, long term preventive effort against radicalization to prevent problems from growing out of hand,” said Dalgaard-Nielsen.

“Police need to prioritize community outreach and long term trust building to try to ensure the collaboration of minority groups and socioeconomically disadvantaged communities in the effort against terrorism.”

Stanford political science professor David Laitin, who recently published the book “Why Muslim Integration Fails in Christian-Heritage Societies,” in collaboration with Claire Adida and Marie-Anne Valfort, said his research found that Muslims faced higher discrimination in the economy, in society and in the political process compared to Christians from similar immigrant backgrounds.

“But there is no evidence that higher degrees of discrimination lead Muslims into the unspeakable acts that members of an inhuman cult are performing in the name of Islam,” said Laitin, who is the James T. Watkins IV and Elise V. Watkins professor in the School of Humanities and Sciences.

“From what we have tragically seen, the attractiveness of the present murderous cult does not derive from everyday discrimination," he said. "Research has shown that it is not the poor and downtrodden who are radicalized in this way; but rather reasonably educated second-generation immigrants from largely secular backgrounds.”

Europe Divided

Laitin said he expected to see many European countries tighten their border controls in response to the Brussels attacks, as well as greater support in the United Kingdom for the movement to leave the European Union in the upcoming referendum.

“The biggest short-term effect, in my judgment, will be the erosion of one of the great achievements of European integration, namely Schengen, which promised open borders throughout the continent,” Laitin said.

“I foresee greater security walls that will come to divide European countries.”

Fighting a Hostile Ideology

Felter said that while it was undoubtedly important to improve intelligence sharing and invest in greater security measures as part of concerted efforts to target ISIS and interdict future terrorist plots, the key to undermining support for and defeating ISIS was combating its perverted version of Islam.

And, he said, that effort would have to come largely from within the Islamic community itself.

“The symptoms may be suicide bombers in airports, but the root cause is this hostile ideology that’s being pushed on these at-risk individuals through aggressive radicalization and recruitment efforts carried out largely via the internet that then inspires them to carry out these self-directed, ISIS-inspired attacks,” Felter said.

“There’s got to be a longer-term effort to address the root causes of this, to discredit and delegitimize the appeal of this ideology that they’re promulgating online and through social media that’s inspiring these young men and women to go off and commit these horrible acts in the misguided belief that it is their religious obligation to do so.”