Belgium was left reeling after three attacks left at least 31 people dead and more than 200 injured March 22. The terror began unfolding during peak rush hour, and ended with at least one suspect still at large. (Deirdra O'Regan/The Washington Post)

One perpetrator was an automobile thief before he got religion, and served time in a Belgian prison on a carjacking charge. Another was an armed robber who once shot a police officer while fleeing from a crime scene.

Others had convictions for burglary, drug-dealing, larceny and assault. Nearly to a person, all had been violent men, long before they became foot soldiers for the hyper-violent Islamic State.

As Belgian police delve into the backgrounds of the men behind Tuesday’s attacks in Brussels, they are encountering a pattern familiar to investigators in Paris and other European cities targeted by the Islamic State: The shock troops used in the terrorist group’s signature attacks are largely men already well known to local law enforcement — not as religious radicals, but as criminals.

As it has done for years in the Middle East, the Islamic State appears to be finding a fruitful recruiting ground among Europe’s street gangs and petty criminals, drawing to itself legions of troubled young men and women from predominantly poor Muslim neighborhoods, U.S. and European officials and terrorism experts say. Some recruits have scant knowledge of Islam but, attracted by the group’s violent ideology, they become skilled and eager accomplices in carrying out acts of extraordinary cruelty.

What we know about the connections between the Brussels and Paris attacks

“Some of these guys are just looking for an opportunity to justify their violence and criminality,” said Ali Soufan, a former FBI counterterrorism official and a consultant to government agencies on terrorist threats. “Now, with ISIS, it is justified — because they can say they’re doing it for God.” ISIS is another name for the Islamic State.

Indeed, some European officials say the perpetrators in the most recent attacks appear to be part of a new wave of recruits that are not “radical Islamists” but rather “Islamized radicals” — people from society’s outer margins who feel at home with a terrorist organization noted for beheading hostages and executing unarmed civilians.

“Their revolt from society manifested itself through petty crime and delinquency,” Belgian counterterrorism official Alain Grignard said in an essay published by the Combating Terrorism Center at West Point. “Many are essentially part of street gangs. What the Islamic State brought in its wake was a new strain of Islam which legitimized their radical approach.”

The thuggish pedigree of the most recent Islamic State attackers was in evidence on Wednesday as Belgian officials revealed new details about the men who carried out the attacks on Brussels’ main airport and subway line. Two of the suicide bombers, brothers Ibrahim and Khalid el-Bakraoui, had spent time in Belgian prisons for violent offenses that included armed robbery and carjacking.

Another member of the Brussels cell, Abdelhamid Abaaoud, leader of the Nov. 13 Paris attacks, also had a lengthy criminal career that included multiple stints in jail, for crimes including burglary and assault. Salah Abdeslam, an alleged accomplice in the Paris attacks who was captured in Belgium last week, had previous convictions for drug-related offenses.

But even before the attacks in Brussels, security officials said that it had become difficult to distinguish the cells of Islamist militants in that city from its criminal networks. Operatives linked to or inspired by the­ ­Islamic State have exploited this overlap to acquire weapons in Belgium and use the nation as a transit point for plots including the attacks in Paris last fall.

Belgian security officials have described the linkages as symbiotic, saying that terror groups rely on their criminal associations to help them hide from authorities, procure explosives and other supplies, and trade information on tactics — even if the two sides don’t share the same objectives or ideologies.

“There are so many links” between criminals and Islamist militants, Belgian Foreign Minister Didier Reynders said at a security conference in Brussels on Sunday. “They are using the same tools. They are using the same cars, the same apartments, the same locations.”

Over the past year, counterterrorism officials and experts in Europe have begun to document a profound shift in the typical profile of terrorist recruits, asserting that the latest arrivals are closer in key characteristics to urban street gangs than religious extremists.

“For them, joining [the Islamic State] is merely a shift to another form of deviant behavior,” said a report released this month by Rik Coolsaet, a professor in Belgium who has studied the foreign fighter flow. Membership in the Islamic State is for many Muslim youths part of a progression that began with “gangs, rioting, drug trafficking and juvenile delinquency,” Coolsaet wrote. “But it adds a thrilling, larger-than-life dimension to their way of life — transforming them from delinquents without a future into mujahideen with a cause.”

The expanding cohort of terror recruits from criminal backgrounds was described by Coolsaet as the “fourth wave” of jihadist terrorism, following cycles including those who flocked to Afghanistan in the 1980s, their elite Middle East expatriate successors who were drawn to al-Qaeda, and finally homegrown radicals who forged their bonds over the Internet.

Religion has plunged as a motivational factor among the latest generation of Islamic State recruits, according to an examination of the group’s terror plots by European security authorities this year. As a result, “it may be more accurate to speak of a ‘violent extremist social trend’ rather than using the term ‘radicalization,’ ” the report concluded.

The prominent criminal element among the networks in Belgium is in contrast to previous generations of terror cells, most notably the roster of al-Qaeda operatives who were based in Hamburg before carrying out the Sept. 11, 2001, attacks.

Few of those militants had a criminal record or even any notable brushes with law enforcement — résumé flaws that al-Qaeda worried would attract scrutiny from law enforcement and risk exposing the group’s elaborate, multiyear plot.

Several members of the Hamburg cell came from middle-class or affluent families. Most spent time in Germany pursuing degrees in highly technical fields such as electrical engineering and chemistry. Their principal bond was a deepening commitment to an extreme interpretation of Islam, which they cultivated during parlorlike discussions at an apartment they took to calling “Dar el Ansar,” or “House of the Followers,” according to the report by the U.S. commission that investigated the 9/11 attacks.

The archetype of this breed was Mohammed Atta, who came from a middle-class family in Egypt, had worked as an urban planner in Cairo and “applied himself fairly seriously” to his studies in Hamburg, according to the report. He went to the trouble of completing his advanced degree before leaving for Afghanistan, where he and others were all but handpicked by Osama bin Laden to lead the plot to hijack airliners and plow them into U.S. landmarks.

The Islamic State is clearly of a different lineage that dates to the group’s earliest days, when it was called al-Qaeda in Iraq. The group’s founder, Abu Musab al-Zarqawi, was a tattooed Jordanian with a long history of criminal violence in his home country.

Zarqawi, a high school dropout with no formal theological training, fashioned the organization in his own image, ignoring Islamic taboos such as the use of suicide bombers when it suited his purposes. His brutality drew harsh rebukes from bin Laden and other al-Qaeda leaders of the time, recalled Nada Bakos, a former CIA officer involved in tracking Zarqawi.

“Zarqawi was never fully accepted into the al-Qaeda brand because he was a thug, and because his logistics network was involved with criminal enterprises,” Bakos said. “These [Islamic State] guys are the same.”