Barcelona club president Joan Laporta has blamed season ticket holders selling their seats for last week's embarrassment against Eintracht Frankfurt.
The Bundesliga side beat Barça 3-2 at Camp Nou to progress to the semi-finals of the Europa League.
A major talking point from the match was that while the away side only had an official allocation of 5,000 tickets, over 30,000 fans Frankfurt fans were present, providing nearly half of the 79,468 in attendance and giving the game a cup-final feel.
Xavi and Laporta were both disappointed with how this shaped the tie, and the latter has defended the club for the fiasco and instead blamed Barcelona season ticket holders for allowing Frankfurt to gain such an advantage.
"The club is not to blame for what happened against Eintracht, but it is responsible," Laporta said at a press conference.
"We didn't sell tickets to German fans beyond the 5,000 away tickets. Sales from Germany weren't allowed and there were many people trying to buy tickets from there. The system of control worked up to a point, but some organised groups were able to breach the mechanisms in place.
"We inherited the system from the previous board. We thought it could work but we have seen that for special games it doesn't.
"Tickets will now be non-transferrable for non-domestic matches and high-risk games. We are working on other measures so that it does not happen again."
Laporta added: "A group of members misused their season tickets, which they ceded to German fans. It is sad because we love this club and it is hard to imagine season ticket holders giving their tickets to rival fans. But this happened and it will be subject to analysis.
"There are around 7,000 members that purchased tickets with the discount code and [those tickets] ended up in German hands. We don't want it to become a witch hunt, but there is a lot of disappointment and will look into possibly taking action.
"We have also detected malpractice from a tour operator, whose contract with the club has now been terminated."