A whistleblower has given evidence about Sepp Blatter that may be "very helpful" in the criminal case against the Fifa boss, say Swiss prosecutors.
Blatter was banned from football for eight years for a "disloyal payment" of £1.3m to Uefa president Michel Platini.
Blatter is also under investigation for alleged criminal mismanagement.
"The Swiss Attorney General's office received valuable information from a testimony that might be very helpful," a spokesman told BBC Sport.
Blatter, 79, is suspected of signing a contract that was "unfavourable to Fifa".
The contract is thought to refer to a 2005 TV rights deal between Fifa and Jack Warner, the former president of Concacaf, the governing body of football in North and Central America and the Caribbean.
Blatter, who has always denied any wrongdoing, is appealing against his ban from football.
However, Fifa's ethics committee hopes to increase his punishment to a lifetime ban.
Blatter won a fifth Fifa presidential election in May, but his 17-year reign at the top of the game's global governing body is set to end in February when the organisation elects a successor.
Platini, who received the money from Blatter in 2011 for work the pair say he did between 1998 and 2002, has also been suspended from football for eight years.
He, too, is appealing against the punishment.
The Swiss criminal investigation against Blatter is ongoing, with German media reports suggesting formal proceedings may start in late 2016.