Mohamed bin Hammam has met investigators for the first time as part of his fight against allegations of financial mismanagement and corruption.
The 63-year-old Qatari, one of the most powerful men in football, has been suspended as president of the Asian Football Confederation (AFC) and from the Fifa executive committee pending the investigation.
Bin Hammam claims the allegations are politically-motivated after he challenged Sepp Blatter for the Fifa presidency last year.
He agreed to meet investigators from the company run by Louis Freeh, a former director at the FBI, to answer allegations against him stemming from a PricewaterhouseCoopers (PwC) audit of AFC accounts during his time as president. Freeh's agency was appointed by the AFC to investigate Bin Hammam following the PwC audit.
It is understood that Bin Hammam, who had a lifetime Fifa ban imposed for bribery overturned in July by the Court of Arbitration for Sport, had a four-hour meeting with Freeh officials in London last week.
After the PwC audit, Bin Hammam ordered his own independent expert's report from London accountants Smith and Williamson into the AFC account, and sources close to him say he provided Freeh investigators with a line-by-line explanation of all expenditure.
The PwC report highlighted a number of personal payments to football officials. Bin Hammam said in a letter to national association leaders in Asia last month these were to people suffering from serious illnesses and to the family of a teenage player who died on the pitch.
Bin Hammam was banned by Fifa for life over allegations he paid bribes while campaigning for president. Bin Hammam denies the accusations.
The ban was overturned by CAS, which said there was insufficient evidence, and the case is now being looked at by Fifa's new ethics investigator Michael J Garcia.