Barcelona coach Xavi Hernandez says his players want to know how at least 20,000 Eintracht Frankfurt fans gained access to the home end at Camp Nou on Thursday as his side were knocked out of the Europa League.

Frankfurt won 3-2 on the night as they stunned favourites Barca 4-3 on aggregate to progress to the semifinals, where they will take on Premier League side West Ham United.

The German side were officially given 5,000 tickets for the game at Camp Nou, but one source told ESPN there were closer to 30,000 inside the stadium to watch the game, with white shirts everywhere you looked among the 79,468 crowd.

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"The atmosphere did not help at all, that's clear," Xavi said in a news conference after his side's elimination. "It affected us and we didn't play well, but that should not serve as an excuse, we also have to congratulate Frankfurt. They were better across the tie.

"It was like a final, with the stadium split. It was disappointing. Even our players commented on it. The dressing room wants to know what has happened. We will seek an explanation.

"This cannot happen when you are playing at home. It hasn't helped, that's clear. It's an error in planning and calculations. The club is going to find out what happened."

Ahead of the game, Xavi had called on Barca supporters to create a hostile atmosphere like the one they faced in Germany in the first leg last week.

But Barca's players were jeered and whistled by the away supporters when they came out to warm up, setting the tone for the night. Barca's ultras -- the supporters who occupy the stand behind one of the goals -- boycotted the first 10 minutes of the second half in protest at so many Frankfurt fans gaining access to the home end.

"Xavi is right, this was a serious mistake," president Joan 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 must be stricter and not allow certain things. I feel ashamed as a Barcelona fan. I apologise. We're processing the information we have and will give explanations."

A brace from Filip Kostic and a long-range strike from Rafael Santos Borre helped Frankfurt into a 3-0 lead on the night as they shocked Camp Nou in front of their vociferous support.

Barca fought back late on with goals from Sergio Busquets and Memphis Depay, but their comeback came too late as their 15-game unbeaten run came to an end. It was their first defeat over 90 minutes since Dec. 8.

"We have to change chip straight away," Xavi added. "I have told the players I am proud, to keep their heads up and that we have to improve and keep believing in what we are doing."

Barca were only in the Europa League because they were knocked out of the Champions League at the group stage and, having already been eliminated from the Copa del Rey and Spanish Supercopa, they are facing a trophy-less season.

They are 12 points behind Real Madrid in LaLiga, albeit with a game in hand.

"Mathematically we can still win LaLiga, yes," added Xavi, who has helped Barca rise to second in the league from ninth after taking over from Ronald Koeman in November.

"I know everything will look bleak now, there will be criticism ... but we have eight finals left and we are thinking of Cadiz on Monday. I hope the fans will be back with us. The primary objective is to qualify for the Champions League."