A raid was carried out at the offices of the French Football Federation in Paris on Tuesday, after the Swiss attorney general made a request for legal assistance in the case against former FIFA president Sepp Blatter.
The Swiss justice authorities say the French Financial Prosecution Office carried out the search in line with the request, and seized documents in connection with a suspected payment of CHF2 million ($2 million) that has been the subject of allegations against Blatter. The Swiss attorney general was present during the search.
The French prosecutor's office confirmed documents useful to the Swiss investigation were taken.
Blatter told the French AFP news agency he was surprised by the search.
He pointed out that the CHF2 million payment was made to a Swiss bank account under an oral agreement. He said neither the European nor the French football authorities were involved.
Criminal proceedings in Switzerland against Blatter opened in September 2015, after a series of scandals and claims of corruption hit the Zurich-based world football governing body.
The investigation centres on allegations of criminal mismanagement and misappropriation of funds.
Blatter and Frenchman Michel Platini, former president of the European football association (UEFA), were suspended from all football-related activities by FIFA's ethics committee last December.
The eight-year suspension, which was recently reduced to six years after an appeal, had to do with the suspicious CHF2 million payment that Blatter made to Platini in 2011 – nine years after Platini had finished working as an advisor to FIFA.
The payment is said to have been made for work completed in 2002 by Platini.
Both men deny any wrongdoing and are appealing to the Court of Arbitration for Sport.
Platini's lawyers said in a statement on Wednesday that the Swiss intervention was a positive step.
"We welcome this new stage because the sooner Swiss justice completes the investigation, the sooner Michel Platini will get out of the news headlines in which he does not belong," the statement said.
swissinfo.ch and agencies