Outgoing FIFA president Sepp Blatter was one of the subjects of the FBI's probe into world football's governing body, according to reports.
FIFA was rocked last week by corruption allegations that resulted in the indictments of 14 people, including a number of high-ranking officials in the organisation.
Blatter was not amongst those arrested by the US authorities, but the long-standing president came under intense scrutiny to stand down from his role following the latest scandal to hit FIFA under his tenure.
However, he was re-elected for a fifth term on Friday, only to shock the footballing world by resigning from his post this afternoon.
It is now being reported by ABC News that the FBI were investigating Blatter, although the authorities declined to comment due to the 79-year-old not being publicly identified as a target of the investigation.
Blatter has been the president of FIFA since 1998, but emergency elections will now take place between September 2015 and March 2016 to find his successor.
