Swiss police are questioning 10 Fifa officials over the controversial award of the 2018 and 2022 World Cups to Russia and Qatar respectively.
Electronic data and documents were seized from Fifa's Zurich headquarters on Wednesday morning, after six Fifa officials were arrested on behalf of a separate investigation from the US Department of Justice.
Swiss authorities have opened criminal proceedings against unnamed persons "on suspicion of criminal mismanagement and of money laundering in connection with the allocation of the 2018 and 2022 World Cups".
Fifa officials are in town for the governing body's 65th annual congress where Sepp Blatter is expected to be elected as president for a fifth straight term. His only challenger, Prince Ali bin al-Hussein of Jordan, said the arrests signalled a "sad day for football".
Swiss and US law enforcement officials are not conducting any joint investigations, but are "coordinating their respective criminal proceedings" in order to obtain all criminally relevant data and avoid any possible collusion.
The Office of the Attorney General of Switzerland explained in a statement that the two raids on Fifa were conducted simultaneously as "a large number of persons involved in allocating the World Cups were currently in Zurich".