An abandoned vehicle near the scene of the worst atrocity in Paris has taken police to Molenbeek – a haven for jihadis

When detectives discovered a grey VW Polo car with foreign licence plates near the Bataclan concert hall after the bloody attacks in Paris, they would have suspected it would offer clues to the assailants who had wreaked such terror on the city hours earlier. Eyewitnesses had, after all, spoken of the black-clad men appearing from cars bearing Belgian plates before they calmly discharged their weapons or set off their suicide belts at sites across the French capital, killing 129 people.

In fact, a parking ticket casually discarded in the small rented vehicle was to tell them much more than they could have hoped – or, indeed, have feared. It had been issued in the Brussels district of Molenbeek.

Immediately the French security services will have been confident of two things: Islamic State was likely to have been behind the attacks, and the security services had dropped the ball.

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A district of derelict warehouses, red-brick terraces and vibrant street life on the canals north-west of the centre of Brussels, Molenbeek was once known as Belgium’s “Little Manchester”. Today it was casually described by one Belgian broadcaster as a “den of terrorists”, where returnees from Syria have in recent years often made their home.

The ordinary daytime strolling atmosphere is entirely comfortable, but it enjoys a reputation for hardline clandestine Salafist cells which the Belgian security services claim to know a lot about but never seem able to deal with. In a country with one of the highest per capita ratios of locally born jihadis fighting in Syria and Iraq of any country in western Europe, this is a hotspot.

Ayoub el-Khazzani, 25, a Moroccan national, who opened fire with a Kalashnikov on a high-speed Thalys train last August had lived there. The 29-year-old French national of Algerian origin, Mehdi Nemmouche, who killed three people at the Jewish Museum in Brussels in 2014 stayed in the district; as did one of those involved in the Madrid bombings in 2003.

It should be of no surprise that President François Hollande was so quick and so certain in his claim on Saturday that the attacks were “an act of war that was committed by a terrorist army, a jihadist army, Daesh, against France”.

After the discovery of that parking ticket, it did not take long for the Belgian prosecutor’s office to open an anti-terrorist investigation, nor for the French border with Belgium to be tightened. Within hours, a car used in the Paris attacks, registered to a French citizen, was stopped there. The three people inside, seemingly returning to Molenbeek, were detained.

Belgian public television network RTBF reported an unidentified source as saying up to three raids had been subsequently carried out in Molenbeek, and police confirmed that three French nationals living there had been arrested. Pictures emerged of one man being taken from an apartment in handcuffs. The Belgian prime minister, Charles Michel, said one of those arrested may have been in Paris on Friday.

“Multiple arrests and search warrants have been executed,” Eric Van Der Sypt, a spokesman for the Belgian federal prosecutors’ office, told reporters. “These operations are still ongoing as we speak.”

The Belgian justice minister, Koen Geens, confirmed the arrests could “be seen in connection with a grey Polo car rented in Belgium”.

At least one of Friday’s attackers, identified by his fingerprints, was a French national from the Paris suburb of Courcouronnes. The man, born in 1985, had a criminal record and had been flagged as an extremist as early as 2010.

But it appears that the Belgian connection is key to what the Paris public prosecutor, at a press conference on Saturday, described as an attack by three cells of jihadi gunmen. It is Molenbeek where a gang of young jihadis coalesced, or was coordinated, under the noses of European’s security services. And it is an international web.

The case of a 51-year-old Montenegrin detained in Germany earlier this month after his car was found to be laden with weapons and explosives was now being looked on in a different light, German police said. The man had been arrested in Bavaria near the Austrian border on 5 November, said Bavarian public radio, with a French address inputted into his satnav.

A Syrian passport, belonging to a man born in 1990 who was not known to the French authorities, had been found lying close to the bodies of two jihadis.

And a second gunman was also found with a passport of a Syrian man born in 1980. He had also not previously been known to French police, but the back story for both men suggested a possible Isis connection. One had been registered in Greece as a refugee; the other appeared to have travelled through the Greek island of Leros.

“On the case of the Syrian passport found at the scene of the terrorist attack, we announce that the passport-holder had passed from Leros on 3 October where he was identified based on EU rules,” the Greek police minister, Nikos Toskas, told reporters, “We do not know if the passport was checked by other countries through which the holder likely passed. We will continue the painstaking and persistent effort to ensure the security of our country and Europe under difficult circumstances, insisting on complete identification of those arriving.”

A source in Athens told the Observer: “We found the serial number and we found the fingerprints and palm prints that are also taken [from every refugee] as part of the Eurodat screening process. All this data is now held in the pan-European data base that all Schengen countries have access to.

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“It is now up to the French authorities to match those fingerprints with the remains of the body [of the attacker] and to announce the identity.

“Either this person passed through Greece posing as a refugee or along the way he bought or stole the passport. It is well-known that Syrian passports are also extremely easy to forge. At this stage either scenario is possible.”

European security officials had feared that jihadis could take advantage of the mass migration, mainly from Syria.

“It is clear now that together with the victims of Islamo-fascism in the Middle East that come as refugees, extreme elements are crossing to Europe,” said the Greek defence minister, Panos Kammenos. In September, Greece’s junior minister for migration, Yiannis Mouzalas, had admitted that it would be “foolish” to discount the possibility of jihadis sneaking into Europe among the refugees. More than 800,000 people have crossed the Mediterranean to Europe this year.

But while there is fear that refugee routes have been exploited by Isis, European security services will be consumed by another thought: what have they missed in Molenbeek?