The Federal Bureau of Investigation has begun an inquiry into the practices of booming daily fantasy sports sites after players and lawmakers made allegations of predatory tactics and questioned the use of inside information, according to players who said they have been contacted by investigators.
The F.B.I. began contacting several prominent competitors in the contests shortly after an employee of DraftKings, one of the two major companies, admitted to inadvertently releasing data before lineups for the start of the third week of N.F.L. games were locked in. The employee, a midlevel content manager, then won $350,000 at a rival site, although DraftKings said he did not have an advantage.