brussels attacks
Brothers Ibrahim and Khalid el-Bakraoui (left and centre) died in the attacks in Brussels when they detonated suicide vests. The third man, identified as Najim Laachraoui, has not yet been caughtCCTV images

As the world reeled from the Brussels attacks on Tuesday, Britain's Foreign Office upgraded Belgium's threat level to four: serious and imminent. It told Britons to remain alert and vigilant and avoid crowded places, recalling the kind of advice usually reserved for the tumultuous nations of the Middle East.

The warning is testament to how Europe has changed in the 15 months since the fanatics behind the Charlie Hebdo massacre first brought their wanton brand of bloodshed to Paris. Across Europe, armed police patrol the streets and residents live in fear of the next atrocity. Meanwhile, hostility towards refugees and migrants from the Arab world has shifted from the peripheries into the mainstream.

This state of affairs has been largely created by a group of 10 men, of which the world has vastly varying amounts of real information. Salah Abdeslam, who was captured last week in Brussels, and Abdelhamid Abaaoud, shot dead in Paris last year, are relatively well known now following Europe-wide manhunts for both following the attacks on 13 November. For others, grainy mugshots are the only information that exists.

That there are links between the men that killed 130 people in Paris last year and the more than 30 killed in Brussels this week is now relatively well established. Experts believe that the men were known to each other and operated if not as one cell then as a number of connected ones. Equally while it is clear that the men had links to Islamic State (Isis/Daesh) in Syria and some had even fought there, the extent to which they took direction from Raqqa or Mosul is less so – especially in the case of Brussels.

The Paris cell:

The Brussels/ Paris cell

Suspected attacks mastermind Abdelhamid Abaaoud (top row, left) fought in Syria and Brahim Abdeslam (bottom row, second from left) took part in the bars and restaurants shootings with a third unidentified man. Samy Amimour (bottom row, second from right).

Omar Ismail Mostefai (bottom row, centre) and Foued Mohamed Aggad (top row, second from right), attacked the Bataclan concert hall. Bilal Hadfi (bottom row, right), and two unidentified men (top row right and bottom row left) blew themselves outside the Stade de France stadium. Salah Abdeslam (top row, second from right) may have conveyed the Stade de France attackers, and is now in custody, as well as Mohamed Abrini (top row, centre) who were seen with Salah Abdeslam on 11 November.

"It is too early to clearly assess the operational link between the Brussels cell and the IS leadership, but in general the IS leadership leaves a lot of operational freedom to its operatives," said Michael Horowitz, a terrorism expert at the Levantine Group.

Paris mastermind Abaaoud is considered the leader of the cell, having been active well before November 2015. He is believed to have been the architect of a thwarted attack on Belgian police officers in the north-eastern town of Verviers in January 2015 which saw two alleged jihadis shot dead and seven arrested. He was able to slip the net and is believed to have then been behind the unsuccessful terrorist attack on a high speed train between Amsterdam and Paris on 23 August last year.

In the immediate aftermath of the Paris attacks, when gunmen and suicide bombers targeted the Stade de France, the Bataclan concert venue and busy restaurants and cafés killing 130 people, Abaaoud was thought to have evaded police and returned to Syria but was later traced to an apartment in the Saint-Denis district of the city where he was shot and killed.

The 27-year-old had been in Syria more than once and boasted in an interview in Daesh magazine, Dabiq: "My name and picture were all over the news, yet I was able to stay in their homeland, plan operations against them, and leave safely when doing so became necessary." His younger brother, Younes Abaaoud, is believed to be fighting in Syria and it has been claimed that Abaaoud met jihadists in Britain on a trip to the UK in early 2015.

Abaaoud was from the same Molenbeek district of Brussels in which Salah Abdeslam and his brother, Ibrahim, lived and owned a bar. Salah has been described as the logistics man of the cell, who personally helped suspected Brussels bomb-maker Najim Lachraoui – currently on the run – travel from Budapest to Brussels after he returned from Syria. He is currently in Belgian custody fighting extradition to France.

With Abdeslam facing trial and Abaaoud dead, attention has now shifted to Ibrahim and Khalid el-Bakraoui – as well as one other attacker that remains at large – responsible for the attacks on the Brussels airport and metro, a Belgian prosecutor told AP. He said Khalid el-Bakraoui, 27, blew himself up at the Maelbeek subway station. The attacks on Tuesday killed at least 31 people and wounded 270, he said.

It was reported on Wednesday that the el-Bakraoui brothers had been known to the police as career criminals and had been linked to shooting in the south of Brussels just a week before the attacks. The link between the brothers and the Paris attacks were made after police raided a flat in southern Belgium and found fingerprints of both Abaaoud and Bilal Hadfi, another Paris suicide bomber. The flat had been rented by Khalid el-Bakraoui using a fake name.

The Brussels cell:

brussels attacks

Brothers Ibrahim and Khalid el-Bakraoui (left and centre) died when they detonated suicide vests. The third man, identified as Najim Laachraoui, has not yet been caught.

The Financial Times reported on Wednesday that both Khalid and Ibrahim el-Bakroui had a history of violent crime, with the latter serving time in 2009 for shooting at police officers and his brother given a suspended sentence in 2011 for a string of carjackings. The link between IS fighters and organised crime is well-established, and goes some way to explain how the men have managed to get hold of weapons relatively easily.

Aside from the fact that the key figures in the attacks in Brussels and Paris knew each other and moved in similar jihadi circles, it is not clear whether they were both coordinated by the same cell. It has been suggested that the arrest of Abdeslam may have prompted fears on the part of the Brussels attackers that he would talk, leading them to carry their atrocity out sooner than they had planned.

"There are key elements that do indeed point to the possibility that the Brussels cell was not as organized as the Isis cell responsible for the Paris attack, or more accurately, that the attack was not as well planned. The fact that the group responsible for the attack took a cab to the airport is one of them. More importantly, in my opinion, the relatively short duration of the attack at the Brussels airport and 'low' death toll – considering the target of the attack – point in the same direction," Horowitz said.

Focus will – and should, said Horowitz – be on Najim Laachraoui, currently on the run in Europe. His DNA was allegedly found on the explosives used during the Paris attack and he represents a key link between that atrocity and Brussels. More importantly, he may also be the key to thwarting future attacks: Isis can find many militants willing to carry out attacks in Europe, on the other hand operatives such as Laachraoui, with this level of bomb-making expertise, are significantly more valuable," he said.

Brussels attack explained: What we know about the deadly blasts that killed more than 30 peopleIBTimes UK