The Federal Bureau of Investigation has begun an inquiry into the practices of booming daily fantasy sports websites after players of the games and lawmakers made allegations of predatory tactics and questioned the use of insider information, according to fantasy players who said they had been contacted by investigators. The F.B.I. began contacting several prominent competitors in the contests, the players said, shortly after an employee of DraftKings, one of the two most prominent daily fantasy 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...