According to the Wall Street Journal, which first reported the news, authorities are investigating whether Under Armour inflated its sales figures.
CFO David Bergman said that the company was “fully cooperating” with the investigation, adding that the company “firmly believe[s] that our accounting practices and disclosures were appropriate”.
Under Armour’s largest market is North America, but it also supplies kit for the Welsh rugby team and Southampton football club.
The company has lowered its full-year outlook in the wake of the news. Having predicted sales growth of between 3-4%, it now expects 2019 revenue to increase by 2% year on year.
It is also currently undergoing a change of leadership, as CEO Kevin Plank steps down and takes up the role of executive chairman and brand chief instead. Current head of operations Patrick Frisk will take over as CEO.
Shares in Under Armour were down 18.4% when they closed on 4 November, leaving the company valued at $7.4bn.