Story highlights

Suspects arrested Friday are among the most sought by European authorities

Other key figures are possibly in hiding in Europe or Syria, or might be dead

Officials concerned about ability of terrorists to get from Middle East to Europe

CNN  — 

The arrests of Mohamed Abrini and Osama Krayem by Belgian police Friday are significant breakthroughs in the battle against ISIS cells in Europe.

But there are perhaps dozens more ISIS members who have reached Europe from Syria, according to investigators.

Abrini and Krayem were taken into custody in separate operations in the Brussels, Belgium, area, according to the Belgian federal prosecutor’s office. Spokesmen for the office also confirmed that Krayem had been using the alias Naim al Hamed.

Abrini and “Hamed” were regarded as the most dangerous suspects among eight men on a European security bulletin circulated in the days after the Paris attacks, and redistributed the day after the Brussels attacks last month.

Abrini had been detected on surveillance video several times in the company of Salah Abdeslam. Investigators suspect Abrini helped transport the attackers from safe houses in Belgium to Paris before the attack. According to the Belgian federal prosecutor, Abrini returned from Turkey to Belgium in summer 2015.

The prosecutor’s office said Friday that investigations are underway to establish whether Abrini was the “third man” at Brussels Airport, and whether Krayem was seen with the metro bomber.

Good news, and bad

The satisfaction that the two most dangerous of the eight in the security bulletin are no longer at large will be tempered by several concerns.

Abrini successfully evaded authorities for several months before being found on the outskirts of Brussels in Anderlecht. And it’s not clear when authorities identified “Hamed” as Krayem. There was no suggestion on the European bulletin issued last month that he was Krayem.

Additionally, there will be concern that Krayem, a Swedish jihadist who had been to Syria, managed to slip into Belgium unnoticed – as had another senior figure in the Paris/Brussels cell, Mohamed Belkaid. Belkaid was shot dead in a police raid days before the Brussels attacks.

Krayem, a resident of Malmo, was on the radar screen of Swedish intelligence services, according to Magnus Ranstorp, a counterterrorism expert at the Swedish National Defense College. He had posted images of himself from Syria with automatic weapons and the ISIS flag, Ranstorp said. The last posting was in January 2015, Ranstorp said.

But then he managed to get back into Europe undetected. Investigators now believe that along with a man using the name Monir Ahmed Alaaj, Krayem reached the Greek island of Leros in September. They traveled through Austria to a refugee camp in Ulm, Germany.

On October 3, Abdeslam collected Krayem and Alaaj at an Ibis hotel near the refugee center in Ulm and drove them to Brussels, investigators said. German investigators believe Abdeslam also picked up a third (unidentified) man.

Alaaj was arrested with Abdeslam in Molenbeek on March 18, days after their hiding place in the Forest district of Brussels was discovered by police.

Others at large?

CNN has seen part of the European security bulletin and confirmed the identity of four men suspected of being part of the Paris/Brussels ISIS group. It’s not known whether they are in Syria or in Europe with false identities – or even whether they are alive or dead.

The four men come from the Netherlands, Germany and Belgium. They are said to have connections to Abdelhamid Abaaoud, the leader of the Paris attacks, or Abdeslam, the only survivor among the Paris attackers.

One of them is a Dutch national of Moroccan descent named Zaid Koullis. Thought to be 21 years old, Koullis was added to the wanted list by European security services days after the Paris attacks.

He is also due to be tried in absentia in Belgium for a role in a terror plot last year coordinated by Abaaoud. The conspirators were planning gun and bomb attacks, but their plans were thwarted by Belgian police in the town of Verviers in January 2015.

The trail for Koullis ran cold after his passport was found at the Verviers house that was raided. Whether he was there or his passport had been used by someone else is still unclear.

Dutch authorities were unable to confirm reports that Koullis was killed fighting in Syria last year, according to Dutch media reports. For now he remains on the wanted list.

The same doubts apply to another suspect on the European bulletin: Yoni Patric Mayne. Mayne has long been on the radar screen of Western intelligence agencies because he accompanied Abaaoud to Syria in January 2014.

Both Abaaoud and Mayne were part of the so-called Zerkani recruiting network – named after Khalid al Zerkani, a Brussels-based veteran of the Afghan jihad who was known as “Papa Noel” because he funded travel to Syria. A Belgian court sentenced Zerkani to 12 years in jail last year. Mayne was tried in absentia and received a 10-year sentence.

According to Belgian court documents, Mayne first tried to travel to Syria in April 2013. He returned home and protested he had only reached Turkey when he was briefly detained.

But he went back. ISIS fighters in Raqqa, Syria, posted a photo on Facebook purporting to show Mayne was dead in March 2014. His mother has said he was killed. The Belgian newspaper Derniere Heure reported that investigators apparently have other pictures of his body than those shared on social media, which were identified by Mayne’s family.

But in sentencing him last July, Belgian magistrates noted there was no proof he had been killed and investigators believe he may still be alive, a source told CNN.

Faking deaths

There is concern among European intelligence officials that ISIS operatives have been faking their own deaths to make it easier to return to Europe to carry out attacks. It was a trick used by Abaaoud in autumn 2014, shortly before he slipped into Greece to coordinate the Verviers plot.

Another man due to be tried in absentia for the Verviers plot is Noureddine Abraimi, Belgium’s national news agency reported. He, too, is on the security bulletin circulated among European law enforcement agencies in connection with the Paris attacks.

Abraimi, 29, is from the Brussels neighborhood of Molenbeek. Like so many of the Paris cell, Abraimi went to Syria in 2014, but it’s unknown whether he is still there. His brother Lazez is in custody, accused of driving Abdeslam across Brussels on the day after the Paris attacks. His attorney said he had no idea Abdeslam had been in Paris.

The Belgian courts are getting used to trying terror suspects even if their whereabouts unknown. The first major trial in Belgium related to jihadi travel to Syria involved 45 defendants but only seven were in court.

In another trial in July – of people associated with Zerkani –14 were tried in absentia. Two have since been killed. Another, Reda Kriket, was arrested in France last month. But 11, all of whom have international arrest warrants, remain at large.

The complexity of the legal process, often involving more than a dozen defendants in terror cases, also means that trials are slow to open. The Verviers raid was almost 15 months ago. The federal prosecutor hopes the trial of those alleged to have been involved will open this month, but the defense wants it delayed until the fall.

Turkish connection

Another of those added to the European security bulletin after the Paris attacks is Huseyin Diler, a 43-year-old of Turkish origin who was living in Germany before leaving for Syria at the end of 2014, German officials said.

Diler traveled from the town of Dinslaken in western Germany with several others, the officials said, a group that became known as the Lohberger Brigade. His younger brother Hasan is thought to have been killed in the Syrian town of Kobane, a source briefed by German officials told CNN.

While in Syria, Diler is believed to have lived with several German and Belgian ISIS recruits, the source told CNN. A photograph posted on social media in 2015 shows Diler with a white beard arm-in-arm with Abaaoud.

Officials also believe there are likely more than a dozen people still at large who provided logistical support to the Brussels and Paris attackers, a source briefed by investigators told CNN.

But connecting the dots, establishing relationships and whereabouts, real names and aliases, remain immense challenges.