On Monday, DraftKings and FanDuel released a joint statement that said "nothing is more important" than the "integrity of the games we offer," but offered few specifics about how they keep contests on the level.
A spokesman for DraftKings acknowledged that employees of both companies had won big jackpots playing at other daily fantasy sites. Late Monday, the two companies temporarily barred their employees from playing games or taking part in tournaments at any other site; they already had prohibited their employees from playing on their own company sites.
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"Both companies have strong policies in place to ensure that employees do not misuse any information at their disposal and strictly limit access to company data to only those employees who require it to do their jobs," the statement said. "Employees with access to this data are rigorously monitored by internal fraud control teams, and we have no evidence that anyone has misused it."
Industry analysts said the episode could leave the leagues open to further criticism that they are too loosely regulated.
"The single greatest threat to the daily fantasy sports industry is the misuse of insider information," Mr. Wallach said. "It could imperil this nascent industry unless real, immediate and meaningful safeguards are put in place. If the industry is unwilling to undertake these reforms voluntarily, it will be imposed on them involuntarily as part of a regulatory framework."
Already, there has been intensifying discussion on social media and among lawmakers over whether daily fantasy games are pushing the boundaries of an exemption in a 2006 federal law that has allowed them to operate. The law prohibited games like online poker but permitted fantasy play, deemed games of skill and not chance, under lobbying from professional sports leagues. The games are legal in all but five states.
But because Congress did not foresee how fantasy sports would explode, one member, Representative Frank Pallone Jr., Democrat of New Jersey, recently requested a hearing to explore the relationship between fantasy sports and gambling. "I really think if they had to justify themselves at a hearing they wouldn't be able to," Mr. Pallone said in a recent interview.
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The data that DraftKings acknowledged was released by its employee, Ethan Haskell, showed which particular players were most used in all lineups submitted to the site's Millionaire Maker contests. Usually, that data is not released until the lineups for all games are finalized. Getting it early, however, is of great advantage in making tactical decisions, especially when an entrant's opponents do not have the information at all.
A spokeswoman for DraftKings said that Mr. Haskell simply made a mistake and that the company was certain he did not use the information improperly. She declined to go into specifics about the safeguards or the company's auditing policies.
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Representatives of both companies acknowledged that many employees of daily fantasy companies were players first and had continued to compete on other sites. Ben Brown, a founder of Daily Fantasy Sports Report, was first to disclose that Haskell had posted the information.
"There's a significant amount of crossover," said Chris Grove, an industry analyst and editor of legalsportsreport.com. "The nature of the industry is so specialized and so new that, at the speed which they grew, they relied heavily on the player population."
Many of these employees set the prices of players and the algorithms for scoring. In short, they make the market.
As daily fantasy sports has blossomed into a multibillion-dollar industry in the past year, DraftKings and FanDuel have become cherished sponsors of M.L.B. and N.F.L. franchises.
Eilers Research, which studies the industry, estimates that daily games will generate around $2.6 billion in entry fees this year and grow 41 percent annually, reaching $14.4 billion in 2020. So high are the potential financial rewards that DraftKings and FanDuel have found eager partners in N.F.L. teams, even as league executives remain staunch opponents of sports betting.
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Jerry Jones of the Dallas Cowboys and Robert K. Kraft of the New England Patriots have stakes in DraftKings, which recently struck a three-year deal with the N.F.L. to become a partner of the league's International Series in Britain, where sports betting is legal. In addition, DraftKings has tapped hundreds of millions of dollars from Fox Sports, and FanDuel has raised similar amounts from investors like Comcast, NBC and KKR.
Adam Krejcik, a managing director at Eilers Research, said early missteps were often part of booming growth in a new and often misunderstood sector like daily fantasy sports. He said whether Haskell, the DraftKings employee, made an innocent mistake or not, the damage was done.
"Certainly does not look good from an optics standpoint, and it strengthens the case for additional oversight and regulation," he said.
Mr. Grove, of legalsportsreport.com, said this may be a watershed moment for a sector that has resisted regulation but now may need it to prove its legitimacy.
"You have information that is valuable and should be tightly restricted," Mr. Grove said. "There are people outside of the company that place value on that information. Is there any internal controls? Any audit process? The inability of the industry to produce a clear and compelling answer to these questions to anyone's satisfaction is why it needs to be regulated."
Disclosure: Comcast and NBC are investors in FanDuel.