ISIL killers freely roamed across Europe
'Man in the hat' left a trail across EU that led to UK and arrests in Birmingham are thought to have been triggered by capture of the Belgian jihadist
As the world looked on, in the summer of 2015, at the sight of crowds of desperate refugees and migrants attempting to make their way to a new life in Europe, debates about the security of the continent's borders took on an urgency perhaps never seen before.
The bloody events in Paris and Brussels only months later would throw the spotlight on to questions not only about Europe's porous external borders but the cherished ideal of free movement within, in the most savage style imaginable. But to Mohamed Abrini, now regarded as a key figure in the planning of both the Paris and Brussels terrorist attacks, what posed a problem for politicians presented a clear opportunity.
While more than a million people attempted to make their way from the Middle East to Europe in the biggest refugee crisis since World War II, the one-time baker and petty criminal from Brussels is thought to have slipped in and out of the region at least twice in a matter of weeks.
Now, as anti-terror investigators home in on the elaborate series of journeys he made through Europe and beyond last summer, the focus is increasingly turning to the UK.
That was thrown into sharp relief yesterday with the arrest of five people from Birmingham alongside a series of anti-terror raids in the city.
They are thought likely to be linked to Abrini's own visit to the city last year as part of a week-long trip, meeting jihadist contacts and, potentially, scoping out new targets for attack.
The 31-year-old, known to friends by the nickname "Brioche" because he once worked in a bakery, was identified last week by Belgian prosecutors as the mysterious "man in the hat" filmed at the scene of the Brussels airport bombings last month.
Born in December 1984 to a family of Moroccan descent, he grew up in the Molenbeek district of Brussels, the neighbourhood in which the terror attacks of recent months, linked to the Islamic State of Iraq and the Levant (Isil), were planned.
A small-time criminal and drug dealer, reported to have up to 50 incidents on his record, he had known Salah and Brahim Abdeslam since childhood. The brothers were at the centre of the Paris terror attacks in which 130 people died in November last year.
His network also extended to Abdelhamid Abaaoud, the ringleader of the Paris attacks, and Najim Laachraoui, who blew himself up at Brussels airport last month. If he appeared more interested in making money than in radicalisation, the death of Abrini's younger brother Souleymane (20) in Syria in July 2014 would have given many pause for thought.
Souleymane is thought to have been fighting in an Isil unit commanded by Mohamed Abrini's associate Abaaoud at the time. Less than a year later, Mohamed Abrini was himself in Syria on what was described as a "quick trip" which would mark the start of an elaborate series of journeys between Europe and the Middle East in the coming weeks. Police believe he flew to Istanbul before making his way from Turkey to Syria in June last year, following a similar route to his dead brother.
He was soon in Turkey again but, rather than making his way directly back to Belgium, it was then that he headed to the UK.
It was the start of a pattern of movements over the next few weeks, travelling through different EU countries where, as a Belgian national, he was free to enter unchallenged.
He is believed to have spent around a week in Britain, reportedly meeting a string of Islamist contacts in Birmingham, Manchester and London.
The visit to the UK was recently confirmed both by French intelligence and by the Interpol president Mireille Ballestrazzi during an interview on a recent edition of BBC's Panorama programme.
Significantly, it is thought that Abrini may also have been scouting potential targets for attacks in the UK. He is reported to have taken pictures of a football stadium. Images of railway stations and shopping centres are also said to have been found on his phone.
And, crucially, he is not the only member of the suspected Molenbeek terror cell to have been in Birmingham, following claims that Abdelhamid Abaaoud is also thought to have travelled to the city.
Continuing his pattern of returning by a separate route, French intelligence sources believe Abrini then boarded a flight from Birmingham to Paris. From there he was picked up by a contact and driven back to Brussels.
By August he was on the move again, but this time the route was through Germany. From there he is thought to have flown to Morocco but little is currently known about his route back. However he returned, by the autumn he was heavily involved in what is thought to have been planning for the Paris attacks. On November 10 and 11, just two days before the atrocities, Abrini is believed to have accompanied the Abdeslam brothers on two separate trips to the French capital, seemingly to rent safe houses.
CCTV footage shows him and Salah Abdeslam in a Renault Clio buying petrol in the Oise region, north of Paris, on November 11. He was also seen in a Belgian service station near the French border the following day in one of the cars used in the Paris attacks.
But as the day of the attacks dawned he is believed to have left Paris for Brussels, reportedly being spotted in a bar in Molenbeek that evening with Ahmad Dahmani, a suspected accomplice who was later arrested in Turkey.
According to both French reports and British sources, by early this year the cell had set their sights on new attacks in the UK as well as the Euro 2016 football tournament in France, which begins in June.
But the arrest on March 18 of Salah Abdeslam prompted an urgent rethink, switching the focus closer to home.
According to Belgian prosecutors, the terrorists had been "surprised by the speed of the progress in the ongoing investigation" and decided to rush the airport and metro station attacks in Brussels, in which more than 30 people died, just four days later. Abrini was caught on CCTV accompanying the suicide bombers Ibrahim el-Bakraoui (29) and Najim Laachraoui (24) to the airport moments before the bombings. But when his own explosives failed to detonate he walked calmly away.
When images of him dressed in a hat and white jacket were circulated he became the most wanted man in Europe.
His own admission, just days ago, that he was indeed the "man in the hat" is now thought to have been the trigger for the recent arrests in Birmingham.