According to Mundo Deportivo, fans of the visiting team Bern violated the rules during the match between Barcelona and Young Boys of Bern.
Fans from the Swiss team successfully broke through Barcelona's strict security measures and secretly brought nearly 20 fireworks into the stands.
In the 75th minute of the game, the visiting fans reached a consensus and lit the fireworks together. Seventeen fireworks with yellow smoke instantly lit up the sky above the Olympic Stadium.
There has been no official response from UEFA to the incident, but the agency responsible for managing European football may review the situation and decide whether to impose penalties on fans who have behaved inappropriately.