The two major U.S. sports fantasy companies defended their businesses' integrity on Monday after an employee used insider information to place bets in the unregulated multi-billion-dollar industry.

The statement from the online companies, DraftKings and FanDuel, came after a DraftKings manager admitted last week he inadvertently released data on National Football League fantasy teams and won $350,000 on FanDuel the same week, the New York Times reported.

DraftKings and FanDuel said they had policies in place to guarantee that employees do not misuse information at their disposal. Access to data is restricted, they said.

"Nothing is more important to DraftKings and FanDuel than the integrity of the games we offer to our customers," they said. "Employees with access to this data are vigorously monitored by internal fraud controls teams, and we have no evidence anyone misused it."

The companies said they were reviewing their internal controls and would work with the fantasy sports industry on the issue.

The business has its roots in informal fantasy games between groups of fans playing against each other for fun over the course of a season. They assembled fantasy professional sports teams and scored points on how players did in actual games.

But companies led by DraftKings and FanDuel have set up online games in which fans pay an entry fee to a website to play dozens or even hundreds of opponents, with prizes that can reach $2 million, the New York Times said.

The Times said that the data released by the DraftKings manager, Ethan Haskell, showed what players were most used in lineups submitted to the site's Millionaire Maker contests.

Normally that data is not released until the lineups for all games are completed. Getting it early is a big advantage, the Times said.

The Times said representatives of both companies had acknowledged that many employees of daily fantasy companies were players initially and continued to compete on other sites.

A DraftKings spokeswoman said that Haskell had simply made a mistake, the Times said. A spokesman for the company did not immediately respond to a request for comment.

The online site Daily Fantasy Sports Report was the first to report that the employee had posted the information, the Times said.