"I'm fairly confident that we will have another round of indictments," said Richard Weber, the chief of the I.R.S. unit in charge of criminal investigations.
The American government's aggressive move shocked the soccer world and led to questions about whether the United States had set out on a mission to topple the leadership of FIFA, which has long been troubled by allegations of corruption. But officials at the Justice Department, the F.B.I. and the I.R.S. said the impetus was criminal activity and organized crime that just happened to occur in the soccer world.
"I don't think there was ever a decision or a declaration that we would go after soccer," Mr. Weber said. "We were going after corruption."
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He added, "One thing led to another, led to another and another."
Still, investigators quickly realized the potential scope of their case. By the time the F.B.I. and the I.R.S. teamed up, an undercover sting operation by the British newspaper The Sunday Times had revealed corruption in FIFA's highest ranks. Reporters around the globe followed with articles about whether soccer's top officials could be bought.
"We always knew it was going to be a very large case," Attorney General Loretta E. Lynch said.
The IRS is known for catching tax cheats, but it is home to some of the nation's most experienced investigators of financial fraud and money laundering. That is especially true since the Sept. 11 terrorist attacks, after which the F.B.I. shifted many of its agents from white-collar crime to counterterrorism. Mr. Berryman, the I.R.S. agent, stayed on the case after it had stretched far beyond tax fraud.
"The case starts off as a tax case against Blazer, but our involvement isn't just on the tax aspect," Mr. Weber said. "Once we get involved in an international corruption case like this one, we use our financial expertise to follow the money."
Neither Ms. Lynch nor Mr. Weber would say what broke the case open, but several law enforcement officials said the earliest success came when Mr. Blazer agreed to cooperate with the government and help prosecutors build a case against other FIFA officials. The Daily News, which first reported that cooperation, said it began in 2011. Working with federal agents, Mr. Blazer surreptitiously recorded FIFA officials in 2012, law enforcement officials said.
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By 2013, when Mr. Blazer secretly pleaded guilty to tax and corruption charges, federal investigators had a solid understanding of corruption in FIFA and had their eyes on top officials such as Jack Warner, court documents show. Mr. Warner was among those arrested this week. Prosecutors say he helped steer the 2010 World Cup tournament to South Africa as part of a $10 million bribery scheme. He has denied the allegations.
The Justice Department's indictment describes a corrupt system in which news media and marketing executives regularly paid bribes to secure the lucrative rights to broadcast and advertise international soccer. In all, prosecutors documented more than $150 million in bribes, and they said American banks had been used in the scheme. That was an important hook allowing prosecutors to bring the case in American courts.
The investigation also benefited from improved cooperation between the Justice Department and international banks. Ten years ago, foreign banks were more willing to help protect illicit money, investigators said. But the crackdown on terrorism financing, along with federal banking inquiries, has made banks increasingly willing to respond when American investigators seek information about account holders and their transactions.
"It's something that for many other countries would be too much to bite off," one federal investigator said when asked why the United States had brought charges.
At the Justice Department, authorities have long known that the case would lead to sweeping indictments. But arresting FIFA officials spread across the globe was an unwieldy proposition. Once the indictment was ready, federal agents recommended making the arrests in Switzerland when FIFA leaders had gathered for an annual meeting.
"Logistically, it just made sense," one investigator said. Any other time of year, the Justice Department would have had to work through several foreign governments to carry out the arrests.
Federal authorities said the timing had nothing to do with the FIFA presidential vote and was not an effort to prevent Sepp Blatter, the longtime president, from being re-elected. Mr. Blatter, who was not charged in the case, won re-election Friday.
Mr. Weber would not identify the remaining targets of the federal investigation or say whether Mr. Blatter was among them. Mr. Weber said he understood that federal investigators, by publicly vowing to rid soccer of corruption, had raised expectations worldwide. But he did not shy away from those expectations.
"We strongly believe there are other people and entities involved in criminal acts," he said.