Barcelona were knocked out of the Europa League on Thursday to Eintracht Frankfurt with around 30,000 German supporters inside Camp Nou.
Officially Barça sent them 5,000 tickets, although they had requested 35,000, and many were able to access the home end to witness their side's historic 3-2 win.
Barça coach Xavi Hernandez and defender Ronald Araujo complained after the game about the scenes in the stands and president Joan Laporta agreed with them.
"Xavi is right, this was a serious mistake," Laporta said. "What happened in the stands is a disgrace that cannot be repeated.
"We have to review the information we have and take measures. It's lamentable. We have to be stricter and not allow certain things. I feel ashamed as a Barcelona fan. I apologise. We're processing the information and will give explanations."
On the game, Laporta added: "We were not able to win and we have to accept it. In football, these things happen. We're sad, angry, but we have to look forward and fight for LaLiga."