Zinedine Zidane's agent says the France captain headbutted Marco Materazzi in Sunday's World Cup final because the Italian made a "very serious" comment.
The moment Zidane headbutted Materazzi to the ground
Zidane appeared to react to something that was said and was dismissed for his violent charge into his opponent.
"He told me Materazzi said something very serious to him but he wouldn't tell me what," agent Alain Migliaccio told BBC Five Live Sport.
Sources in France say it is believed Materazzi insulted Zidane's family.
Materazzi himself has not spoken publicly about the incident except to deny a claim by Paris-based anti-racism group SOS Racism, made on Monday, that he had called Zidane "a dirty terrorist".
"It is absolutely not true, I did not call him a terrorist. I'm ignorant. I don't even know what the word means," the Italian news agency Ansa quoted Materazzi as saying after the Italian team returned to Rome.
"The whole world saw what happened on live TV," he added.
Migliaccio, who spoke to Zidane early on Monday morning, added that Zidane did not elaborate on what Materazzi said.
"I don't know. Zinedine didn't want to talk about it but it will all come out in the next week," he said.
"He is a man who normally lets things wash over him but on Sunday night something exploded inside him.
"He was very disappointed and sad. He didn't want it to end this way."
Zidane appeared in a happier mood during his meeting with Chirac
The France squad arrived back on Monday and went straight to a lunch reception hosted by President Jacques Chirac.
Zidane was also in attendance and was cheered by fans who gathered to welcome the team home.
Offering his support, Chirac said: "Dear Zinedine, in such a hard and intense moment for you, I would like to express the whole nation's affection and admiration for you.
"You are a virtuoso, a genius of football and an exceptional human being. That is why France admires you."
Meanwhile, France defender William Gallas was adamant Zidane was provoked into the headbutt.
"I didn't see it but sometimes football is not only with the ball," the Chelsea star told BBC Sport.
"Sometimes when you have a player who is very clever and says something to you then maybe you are angry and want to 'kill' that player.
"Maybe, and I say maybe, he has made a mistake."
Materazzi refused to answer questions afterwards, though Materazzi's agent Claudio Vigorelli said: "I've known Marco for a long time and I don't think he is capable of provoking a player. He is a good boy."
But Gallas interpreted Materazzi's silence on the matter as an admission of guilt by the Italian.
"I don't know what he said. You'll have to ask him but he's gone. I know why he's gone because maybe he's done something wrong."
On his way back to the dressing room after his 110th-minute dismissal, Zidane passed the World Cup trophy.
It brought a sad end to his glittering career and his misery was compounded when France went on to lose 5-3 on penalties.
"Zidane didn't say anything," said Gallas. "He was very disappointed for everybody - for the team and himself.
"I'm very sad for him. It was his last World Cup and everybody wanted to win for him.
"We deserved to win but that is life. I think we played very well and better than Italy but sometimes football is very strange.
"Everybody is disappointed. I hope we will maybe win next time."
Materazzi's father believed his son was the real victim in the incident.
"I spoke to my son briefly after the game," said Giuseppe Materazzi.
"Marco had told me he had been provoked. It's as though they have something against him each time.
"In the past two years, Marco has only been on the receiving end and the injuries he has sustained are proof of this.
"I don't want to be controversial, but there are things that have to be said instead of just pointing the finger and putting a black mark against someone."