RIO DE JANEIRO (AP) ― FIFA expressed “substantial doubts” Wednesday about a German magazine’s claims that a World Cup game could have been fixed and asked it to provide evidence to back up its report.

FIFA said it has asked Der Spiegel to provide details of all its conversations with convicted match-fixer Wilson Raj Perumal to prove its claim that Cameroon’s 4-0 loss to Croatia may have been fixed.

“The article has put the integrity of FIFA World Cup matches in question, which is a serious allegation,” FIFA director of security Ralf Mutschke said in a statement read out at a daily briefing by spokeswoman Delia Fischer.

FIFA said it had no indication from betting markets that any of the 56 games so far at the World Cup had been manipulated. It said it had “substantial doubts” about weekly magazine Der Spiegel’s claim that Perumal told it in an online chat hours before the Cameroon-Croatia group game on June 18 that he knew what was going to happen.