The allegations have also raised questions for the unregulated industry, including whether or not professional fantasy players have an unfair advantage over regular players. Sports Business Daily found that over the first half of this year’s Major League Baseball season, 91 percent of player profits in daily fantasy sports were won by just 1.3 percent of the players. In fact, the top 11 players on average paid $2 million in entry fees and made $135,000 in profit while accounting for 17 percent of all entry fees.

Many of the professionals use automated processes that let them change hundreds, if not thousands, of lineups in seconds, a decided advantage when last-minute changes are made in the lineups of real football, basketball or baseball teams.

The Wall Street Journal first reported on the F.B.I. interviews of players.

Last week, the New York attorney general opened an inquiry into the prospect that employees of DraftKings and FanDuel won lucrative payouts based on information not available to the public. In addition, three class-action lawsuits have been filed alleging fraud — the most recent in Louisiana, where the operation of daily fantasy sports sites is prohibited. The plaintiff, Artem Genchanok, a New Orleans resident, said he had paid entry into DraftKings and FanDuel contests and deposited money on their websites despite the prohibition.

Lawmakers have been intensifying calls for federal regulation and inquiries into the industry.

In Washington, Representative Hakeem Jeffries, Democrat of New York and a member of the House Judiciary Committee, this week called on the panel to examine “whether permitting a multibillion-dollar industry to police itself serves the best interests of the American people.”

Senator Robert Menendez and Representative Frank Pallone Jr., Democrats of New Jersey, held a news conference Tuesday outside MetLife Stadium, the home of the Giants and the Jets, to reiterate their calls on the Federal Trade Commission to implement safeguards and ensure a fair playing field.

And Senator Richard Blumenthal, Democrat of Connecticut, on Monday formally called for a federal investigation into any deceptive or fraudulent practices at daily fantasy sports leagues.

“Consumers had no foreseeable knowledge that these companies were facilitating employees’ use of proprietary data to provide themselves with an advantage when playing users on their rival site,” Blumenthal said in a letter to the Justice Department and the F.T.C. “If employees are using insider information to unfairly advantage themselves over others, this may constitute fraud regardless of any other federal or state gambling statutes.”