Blatter was banned from football for eight years for making a "disloyal payment" of £1.3m to Uefa president Michel Platini, and is also under investigation by Swiss state lawyers 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. But Fifa's ethics committee hopes to increase his punishment to a lifetime ban.
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He 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 come to an 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 or early 2017.
A whistleblower has given evidence on Sepp Blatter that may be "very helpful" in the criminal case against the banned Fifa president, say Swiss prosecutors.