BEIRUT — Europe is facing an unprecedented onslaught of terror at the hands of militant groups whose twisted roots have been nurtured on the battlefields of Syria and Iraq. European nationals have swelled the ranks of fighters on foreign soil, but the increase in attacks and near-constant threats from the Islamic State group have made it difficult to establish what came first: homegrown European terror networks or imported militants from overseas conflicts.

Though the terrorist organization, also known as ISIS, has claimed responsibility for most of the recent attacks in Europe, the European terror cells that either facilitated or were linked to the operations predate the militant organization. Before the violent attacks last November in Paris, any large-scale, sophisticated operation like the triple bombing Tuesday in Brussels would have been considered the work of al Qaeda, whose first priority was always to carry out explosive attacks against the West. Until last year, ISIS was almost exclusively focused on expanding its so-called caliphate in Iraq, Syria and (now) Libya. But in Europe, allegiances are now blurred. ISIS and al Qaeda cannot easily be distinguished, and newly trained fighters returning from foreign battles are partnering with seasoned European militants.

“International aviation sounds more like an al Qaeda-style attack, but at the same time, [the airport and metro stations] are very obvious terrorist targets,” Rafaello Pantucci, director of the Royal United Services Institute, told International Business Times. “I think we’re drawing lines too cleanly between the two groups, but at the end of it, in the world of radical individuals, there is overlap between the two groups.”

As travelers were queuing up to check in for their morning flights in the departures lounge of Brussel’s Zaventem airport Tuesday morning, explosions rocked the area when two bombs — one in a suitcase, the other a suicide bomb — were detonated. Less than an hour later, a third explosion hit the Maelbeek metro station, close to the city's European Union buildings. ISIS later claimed responsibility for the attacks, using a similar statement to the one that linked the group to the November attacks in Paris.

This is the first large-scale attack ISIS has claimed in Belgium, but the European country is no stranger to the terror group. Roughly 560 Belgians have traveled to Iraq or Syria to fight alongside militant groups since 2012, and about 75 percent of them joined ISIS, according to Pieter Van Ostaeyen, an independent analyst who focuses on foreign fighters.

Among those fighters were Belgian nationals Abdelhamid Abaaoud and Brahim Abdeslam. They traveled to Syria in 2013 and 2014, respectively, before returning to Belgium with an agenda. Both men were killed when they carried out suicide bombings in the November Paris attacks.

Abaaoud and Abdeslam grew up in Molenbeek, a densely populated neighborhood in Brussels where Muslims make up roughly 39 percent of the population. In 2010, before the start of the Syrian Civil War and ISIS’ declaration of being a so-called caliphate, Abaaoud and Abdeslam’s brother Salah were arrested for attempted robbery.

Salah, 26, is a French citizen, born in Belgium and accused of being one of the leading orchestrators of the deadly Paris attacks. On Friday, Belgian authorities arrested him, along with two other men, in Molenbeek. Belgian authorities charged him with participating in a terrorist murder and being affiliated with a terrorist organization.

His charges did not specify to which terrorist group he belonged, but his connections and location point to a European terror network that has been active for years, particularly in Molenbeek.

The main recruiter in the area was Khalid Zerkani, a 42-year-old Moroccan. He has ties to both Syria and Somalia and was arrested in 2014 after being accused of sending at least 45 Belgian citizens to Syria between 2012 and 2014, according to Ostaeyen.

“His recruits not only traveled freely to Syria; they also came back as they liked,” Ostaeyen wrote.

The Abdeslam brothers, Abouaad and one other militant involved in the Paris attacks were part of this network. Belgian authorities are now on the hunt for yet another one of the Zerkani network’s recruits.

“The network has a high level of operational security,” Justin Crump, CEO of Sibylline Ltd., an international risk analysis firm and specialist on jihadist groups, told IBT. He added that the network is likely “self-inspired and run without direction from Syria. They were given a mission, and they had to carry it out.”

Dismantling Zerkani’s network may prove to be even more complex, as his recruiting skills were not limited to Syria or to ISIS.

“Every time he was staying at someone’s house, the proprietary left either for Somalia or for Syria,” the judge at Zerkani’s trial said.

The Belgian court’s investigation also showed he was connected to an infamous Tunisian jihadi Seifallah Ben Hassine, who fought with terrorist groups in Afghanistan in the 1990s. Also known as Abu Iyadh, he is the founder of Ansar al-Sharia in Tunisia, a militant group with ties to both al Qaeda and ISIS. Until he was killed last year, he was a major recruiter for Tunisian and Libyan fighters hoping to go to Syria.

In 2012, Zerkani had several meetings in Molenbeek with Belgian national Youssef Bouyabarem, who was 24 years old at the time. The following year, Bouyabarem attempted to travel to Somalia to fight with al Qaeda’s linked terror organization al-Shabab, of which his brother was already a member. He failed to get to Somalia, and then, with Zerkani’s help, attempted to join the fight in Syria but never succeeded.

Zerkani was sentenced to 15 years in jail, but even with the key recruiter behind bars, “Belgian authorities are dealing with a very large network, and they haven’t been able to disrupt it all,” Pantucci said.