The boos that echoed from three-quarters of Leoforos Alexandras Stadium could not blank out the message held up by the small but inescapable minority. As Panathinaikos and the visiting AEK Athens entered the field for Monday’s derby a 40-metre banner was unfurled by the home ultras group, Gate 13, to a backdrop of billowing green and blue smoke. “Free the boys,” it read, and the reaction from all sides left no doubt that its meaning had resonated.

While Gate 13’s fellow supporters aired their disgust, the AEK players headed back to the touchline. This was a shocking affront to one of their own and they knew it. In August, more than 100 supporters, 96 from the notorious Bad Blue Boys group that follows Dinamo Zagreb, were detained in the Greek capital in connection with violence that led to the fatal stabbing of Michalis Katsouris, a 29-year-old AEK fan. A further nine were arrested in Zagreb last week.

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Katsouris had been killed after a premeditated attack on AEK fans by the Croatian hooligans, who had apparently been assisted by friendly elements from Gate 13. The insignia held up before kick-off appeared to be excusing all of it.

AEK stood in a huddle by the technical area while their opponents lined up in formation. Their club had been warned that such a display was likely and had already made a statement requesting it be stopped.

The scene could have been even more distasteful: if a message in support of alleged violent criminals was not enough, Gate 13 is understood to have been dissuaded from bringing a second banner that directly focused on Katsouris into the ground. The one that got through remained up in its entirety for three minutes and AEK, who would go on to win 2-1, returned to the pitch on its removal.

“A shameful and insulting banner to the memory of Michalis Katsouris,” was how AEK, who did not respond to a request for further comment, publicly described the scene. It had brought into fresh focus a case that caused shock waves across the region seven weeks previously.

Uefa had banned Croatian fans from attending the first leg of the Champions League third qualifying round tie between AEK and Dinamo, with problems already having been anticipated. But up to 200 members of the Bad Blue Boys, a group with long-term links to Croatian nationalism and the far right, managed to evade the Greek authorities and arrived in Athens on the eve of the game with the express intention of targeting home supporters. Dinamo condemned their actions.

Was it a politically motivated attack or an act of convenient brutality? The Bad Blue Boys and Gate 13 have an established relationship: a congratulatory message from the former in 2021 to mark Panathinaikos’ 113th birthday remarked that they “share the same enemies who have the same corrupt ideology”. The Croats knew they would be greeted by willing accomplices upon arrival. But they would also have been perfectly aware that AEK’s fanbase is to a significant degree antifascist, a position that stems from the club’s founding by refugees in the 1920s. It was a clear opportunity to silence a body with opposing ideals.

It was not the only recent flashpoint involving Croatia, who were placed under the latest of several Uefa investigations on 13 September when a flag of the fascist Ustase regime was displayed at the national team’s Euro 2024 qualifier with Latvia in Rijeka. There is a sense that far-right activity, never purged from Europe’s stadiums but arguably less emboldened over the past half-decade, is rearing its head in pockets of the continent again.

“During the Covid-19 pandemic a lot of these things slowed down, fell apart or lost their focus,” says Piara Powar, executive director of the anti-discrimination Fare Network. “The war in Ukraine also threw things asunder and many of these groups struggled to respond to it. What we’re seeing now is a sense of business back to normal: these organisations coming back to full strength after Covid, together with having understood the war and made an accommodation for it.”

Powar notes that, in 2023 alone, Fare Network has observed far-right activity from fans of clubs and national teams in the Netherlands, Spain, France, Ukraine, Poland, Croatia, Bulgaria, Italy, Romania, Slovakia and Montenegro. The list is unlikely to be exhaustive. Even so, few such incidents bring tragedies of the level that claimed Katsouris’s life and it asks tough questions about the balance of responsibility between football authorities and state entities. Uefa acted quickly to ban Dinamo fans from the rest of their European away games this season but grappling with the wider issue is more difficult: far-right violence in football is at once a transnational problem given life by specific local conditions.

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In Greece, that included policing that was, according to claims in some quarters, wilfully negligent at best. It is an age-old tale and so is the level of ultras’ influence that allowed Gate 13 to reignite tensions on Monday night.

“The security each club has is not willing, capable or able to do anything about it: they’re more like concierge,” says Giannis Alafouzos, the Panathinaikos president, although it is not clear why punishments could not have been retrospectively imposed on individuals.

Alafouzos places the ultimate responsibility on governments. “Violence is more an expression that we’re doing something wrong in our countries, and it’s less that football can do something about it,” he says. “As a club you can work to show you have a different approach but you need the cooperation of the state.”

Far-right violence in Greece has complex roots but was watered down upon the collapse of Golden Dawn, a neo-Nazi party whose popularity peaked in the first half of the last decade and whose leaders appropriated football with some success.

There are some concerns that the present climate, in which the extreme right parties Spartans, Greek Solution and Niki all entered parliament this summer, may cohere elements that had become disparate. Gate 13’s widely condemned and intensely provocative intervention, whether overtly politicised or a deeply misguided show of brotherhood, hardly eases such fears.

The detained supporters may be held for up to 18 months under Greek law, although the identity of Katsouris’s killer remains unknown. Football, in the meantime, will remain an all-too convenient vessel for society’s darker forces.