Barcelona manager Xavi complained after his side's Europa League quarter-final defeat on Thursday that there were too many Frankfurt fans in attendance at Camp Nou, saying the club is looking into it.
The Blaugrana lost 3-2 (4-3 on aggregate) to crash out of the competition as a strong run of form came to an end.
White-shirted Frankfurt fans stayed long after the final whistle to cheer for their club.
"I was expecting 70,000 or 80,000 cules here but it wasn't like that," Xavi told reporters. "The club is checking what happened."
Ronald Araujo added to Movistar+: "I'm surprised there are so many German fans in the stadium. The club will have to look at that."
Later, club president Joan Laporta said: "We are worried about the fact that today a big shame happened which can't happen again. We have a large part of the information at hand of what happened, we need time to process this and we will take the appropriate measures but it's shameful what happened."
Frankfurt supporters made themselves felt even before the match, when they marched together through the streets of Barcelona.
They were then treated to a dominant performance from Frankfurt, who led 3-0 until a pair of stoppage-time consolation goals from the Blaugrana.
Frankfurt will now try to invade London in a similar manner when they take on West Ham in the semi-final.