Police officer and soldiers on security duty inside Galerie de la Reine in Brussels, Belgium, 22 November 2015. (EPA/STEPHANIE LECOCQ)

LONDON — When Danish police officers shot a suspected drug dealer earlier this fall in Copenhagen, they assumed the investigation would lead to organized crime. But soon after, the Islamic State claimed the 25-year-old had been a “soldier of the Caliphate.” That wording has been frequently used by the group in recent months, including after the Orlando attack on a gay club in June.

It remains deeply unclear whether the Islamic State has direct links to such attackers or merely piggybacks off their acts in efforts to claim a wider reach. Experts tracking militant trends, though, are digging deeper.

Criminals turning into terrorism suspects is becoming an increasingly common pattern in Europe, according to a new report released Tuesday by the International Center for the Study of Radicalization and Political Violence (ICSR) at King's College London.

The research center collected extensive data on 79 recent European militants with criminal pasts. Their profiles suggest that the networks and underworlds of criminals and terrorists are increasingly merging. Rather than refusing criminals access to their group, the Islamic State has encouraged them to join — a break from militant groups such as al-Qaeda that sought tighter control over their ranks.

“A lot of the people we looked at had no problems being criminals and terrorists at the same time. Many were drinking and smoking literally until days before they carried out attacks,” said Peter Neumann, director of the ICSR and an expert on radicalization.

According to his findings, 80 percent of those involved in recent terrorism plots had criminal convictions. The report concludes that experts and government officials around the world need to rethink their understanding of how individuals radicalize and join militant terrorist groups. Prison incarceration increasingly connects criminals and individuals who support militant groups such as the Islamic State.

The researchers fear that extremists with criminal pasts could plan large-scale attacks more easily, because of underworld contacts and previous experience dodging the law. About 40 percent of all recent terrorism plots in Europe were financed through “petty crimes” such as theft or drug dealing, for instance.

For European officials, the possible crime-terrorism nexus could expand calls for a stronger collaboration between local authorities fighting organized crime and national or international counterterrorism officials. In the European Union, experts have frequently complained that national authorities of the 28 member states only sporadically share information. One of the main suspects in the Paris terrorist attacks in November could escape France because he was only listed as a petty criminal in international databases but not as a potentially dangerous extremist, because of privacy concerns.

Belgian soldiers keep guard outside the town hall in Molenbeek in April. (Yves Herman/Reuters)

Some recent terrorist attacks, though, are thought to have been prevented by improved information sharing. On Monday, a 22-year-old Syrian bomb-plot suspect was arrested in the eastern German city of Leipzig after another E.U. member state alerted German authorities, providing information on a possible Islamic State link.

Report co-author Rajan Basra said that many Islamic State members found themselves being marginalized in European countries before joining the group. “Often, we find breaks in their families,” Basra said.

Joining militant factions offers some criminals a way of supposedly seeking redemption, Basra said. Contrary to al-Qaeda, newer militant groups have legitimized crime and even published propaganda directly targeting criminals in some instances. “Sometimes people with the worst pasts create the best futures,” read one recent propaganda poster by an Islamic State-affiliated group that was republished in the ICSR report.

“The profiles and pathways in our database suggest that the jihadist narrative — as articulated by the Islamic State — is surprisingly well-aligned with the personal needs and desires of criminals, and that it can be used to curtail as well as license the continued involvement in crime,” the report authors contend.

The authors also quoted some members of militant groups describing the approach in a Venn diagram of crime and terrorism. “I really don't know what's gonna happen. Maybe I'm gonna die there,” one member of a militant group was quoted as explaining. “So what? Because, you know, some people have died of my hands.”

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How a London soccer club tries to save kids from the Islamic State