The federal government joined the bitter debate over college football's postseason system.

The Justice Department sent a letter Tuesday to the National Collegiate Athletic Association stating that "serious questions continue to arise" over whether the Bowl Championship Series—the sport's much-criticized method for choosing a champion—complies with antitrust laws.

The letter also asks why major-college football doesn't have a playoff, when so many other college sports do; what steps the NCAA has taken to create a playoff; and whether the NCAA has determined that aspects of the BCS system are unfair.

The letter, addressed to NCAA President Mark Emmert, is the latest and most dramatic event in a series of recent developments buffeting the BCS. In the past six weeks, the Fiesta Bowl fired its chief executive, John Junker, alleging financial improprieties. A group of law and economics experts wrote the Justice Department asking it to probe the BCS on antitrust grounds, and Utah Attorney General Mark Shurtleff said he intended to file an antitrust suit against the BCS.