Both Nice and Cologne on Friday condemned the violence at their Europa Conference League game in southern France that left one fan in critical condition and dozens of others injured, as UEFA opened disciplinary proceedings against both clubs.
"We condemn the horrific scenes that took place before yesterday's game in Nice in the strongest possible manner," Cologne club president Werner Wolf said in a statement on the team's Twitter account.
Later in the day, his Nice counterpart, Jean-Pierre Rivere blamed some of the 8,000 Cologne fans, while acknowledging security at the Allianz Riviera stadium had broken down.
"When you live it live, it's terrible. The next morning, you relive it, it's even worse," said Rivere.
"We have dedicated an area for screening," he said. "But when we opened, they broke everything."
He said there were "flaws everywhere", especially in the collaboration between clubs.
An hour before the scheduled kick-off, several hundred hooded fans in Cologne colours invaded the Nice stands.
Clashes erupted, with some fans armed with chairs or iron bars torn from the stadium, and a total of 32 people were injured, including two police officers and a steward.
Four were hospitalised, with one – a Parisian who had been among the German supporters — in a critical condition after falling in the mayhem.
UEFA said it would investigate Nice over the behaviour of their fans, but also look into possible breaches of its security and safety regulations by the home club.
Rivere complained that the design of the stadium, which is owned by the city of Nice, made it hard to separate fans.
"The stadium is beautiful, but its design prevents the sectorisation of the stands," he said.
UEFA said Ligue 1 side Nice will also have to answer for shortcomings in the "identification of persons responsible".
"It is not possible that they are not identified, especially the French," said Rivere, blaming "some Parisians" who "were not at their first incident".
The German visitors will, like Nice, be investigated for "crowd disturbances", "throwing of objects" and "lighting of fireworks".
"These hordes, these street fighters have no business in a stadium or around it," said Rivere. "If we don't get these people out of the stadiums, we will never solve the problem."
Cologne would "do everything in our power to clear up what happened and go with full consequence against those who chose violence", Wolf said.
"We owe that to our thousands of peaceful fans and football in general."
In comments on the Cologne website, sporting director Christian Keller said he was "speechless".
"We wanted to celebrate a big, peaceful footballing fest," Keller said, adding that instead the events had only led to "suffering".
The violence prompted a delay of nearly an hour to the start of the game.
The Group D match eventually kicked off, ending in a 1-1 draw.
In August last year, a Ligue 1 game between Nice and Marseille was abandoned after a pitch invasion by fans, which was followed by a fight between supporters, players and staff of both teams.
In 2017, the kick-off of a Europa League match between Cologne and Arsenal was delayed by an hour after police made arrests at the Emirates Stadium as German fans occupied home sections of the ground.