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.
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 mother.
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."
http://news.bbc.co.uk/sport2/hi/football/world_cup_2006/teams/france/5164094.stm