The Federal Bureau of Investigation (FBI) are investigating Fifa president Sepp Blatter in connection to a $100 million (£66.2 million) bribes scandal involving a global sports marketing firm. The company, ISL, paid money to officials including former Fifa president Joao Havelange and ex-Fifa executive Ricardo Teixeira – who is Havelange's son-in-law.

ISL, now defunct, were subsequently handed television and marketing rights throughout the 1990s. Havelange and Teixeira resigned after a Fifa ethics committee report revealed they had taken a series of bribes over an eight-year period.

Blatter, 79, has always denied knowing about the bribes, but a letter obtained by the FBI may prove that this was not the case. The letter is part of an FBI request to Swiss authorities for help in their probe into Fifa.

The letter, obtained by the BBC, apparently written by Havelange, says that Blatter had "full knowledge of all activities" and was "always apprised" of them. A new Panorama documentary based on the latest FBI probe is set to be aired tomorrow night.

Both Havelange and Teixeira were found guilty by Swiss prosecutors of receiving bribes from ISL in 2010. Blatter later told an ethics committee that he did not know about the convictions and he was absolved of any wrongdoing.

Damian Collins MP a prominent campaigner against Fifa's conduct, told the Panorama programme: "You have to ask yourself why did he seek to protect these people, and not just protect them but allow them to continue to play an active role in some of Fifa's most important decisions?"

Blatter is set to stand down as head of Fifa in February 2016 and he is currently serving a provisional 90-day suspension from the organisation. He faces a hearing with the body's 'ethics judge' later this month.

On December 3 two Fifa officials were arrested in a dawn raid at the Baur au Lac hotel in Zurich ahead of a Fifa executive committee meeting. The two arrests were made over allegations of taking money for selling market rights for football tournaments in Latin America – including World Cup qualifying matches. In May, Swiss police arrested seven Fifa officials.