On April 15, 2011, thousands of Americans who played online poker woke to find three popular websites shut down by the FBI and millions of dollars in players’ accounts frozen. The online poker industry survived its “Black Friday,” as the day became known, as the Obama administration declared online gaming legal later that year in states that approved it. But the aftermath should serve as a looming worst-case scenario to the millions of Americans who play daily fantasy sports, legal experts say, as many of those players who had money saved in online poker accounts had to wait years before…

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…