In the indictment, prosecutors seemed to skirt the issue. They based parts of their prosecution on state laws in New York and elsewhere that prohibited unlicensed gambling, including poker. Legal experts said the prosecutors needed to rely on such prohibitions as a foundation for going after the claims of money laundering and fraud. But Mr. Walters said this strategy might face challenges. He said it was not clear that the state laws applied to foreign-based gambling operations, given that under federal law, international commerce was regulated at the federal level.

“This appears to be a precedent-setting case,” Mr. Walters said. “It will be the first time the Department of Justice takes on the looming question of whether federal law prohibits online poker.”

Opponents of online gambling have been trying to figure out for years how best to police casinos that can be located abroad but reach easily into American homes. According to statistics provided by the Poker Player’s Alliance, an advocacy organization led by former Senator Alfonse D’Amato of New York, seven million Americans play online for money once a month.

In a statement, Mr. D’Amato criticized the prosecution. “We are shocked at the action,” he said, adding, “Online poker is not a crime and should not be treated as such.”

ComScore, a company that measures Internet traffic, said that in March, Full Tilt Poker had 2.6 million visitors from the United States, PokerStars had 1.9 million and Absolute Poker had 1.3 million. ComScore also reported that 1.4 million people visited Ultimate Bet, a site that the federal indictment says joined forces last year with Absolute Poker. Those were the nation’s four most popular poker sites, ComScore said.

On Friday, visitors to those sites were met with a message that begins: “This domain name has been seized by the F.B.I.” The government used the same controversial tactic of seizing domain names in actions last year against sites accused of copyright violations. Losing a domain name can be costly for a company that has invested in promoting it as the main route to its site. But the tactic can also be of only temporary effectiveness, because the company can switch to a new Web address that is outside the reach of law enforcement in the United States — one that, for example, does not end in .com.

According to the indictment, the fraud and money laundering scheme evolved from deception to bribery. Initially, after the enactment of the 2006 law aimed at limiting payment processing for online gambling, the defendants sought to dupe banks by creating fake companies, according to the indictment.

But the indictment says that as financial institutions got wise to those efforts, several defendants sough to persuade smaller banks to process the transactions by making multimillion-dollar investments in them.