U.S. Attorney General Loretta Lynch declined to comment on FIFA president Sepp Blatter's resignation or whether he was himself under investigation.
"It's an open case and so we will now be speaking through the courts," she told reporters in Riga, Latvia, after a meeting with EU justice ministers.
ABC News reported on Tuesday that the FBI and U.S. prosecutors had launched an investigation into Blatter's presidency and hoped to use those indicted to provide information that could lead to charges against their superiors -- much as federal agents would in a mob racketeering case.
The FBI declined to comment because Blatter has not been publicly identified as a target of the investigation.
"We may not be able to collapse the whole organisation, but maybe you don't need to," one of the sources said.
Meanwhile, Blatter was back at work at FIFA headquarters on Wednesday as the worst corruption crisis in the governing body's 111-year history continued to unfold.
Interpol added six men with ties to FIFA to its most wanted list, while South African officials denied they made a $10 million bribe to secure the 2010 World Cup. Russia and Qatar also issued statements about their continued planning toward the respective 2018 and 2022 World Cups.
Blatter spoke to FIFA staff for about 10 minutes on Wednesday morning, returning to the same auditorium where he delivered his resignation speech a day earlier. Staff described him as being emotional, and said he received a standing ovation.
U.S. Attorney General Lynch said that the U.S. hopes that FIFA "will be able to move forward in a way that is supportive of its goals, which are the promotion and regulation of a truly wonderful sport."
Asked about allegations of suspicious payments related to the awarding of the 2018 World Cup to Russia and the 2022 tournament to Qatar, she said matters related to those bids were part of the Swiss investigation.
Russia, meanwhile, said Blatter's resignation hasn't affected the country's plans to host the 2018 World Cup.
Russia, which was a staunch supporter of Blatter, says it will continue to work with him until he steps down and a new election for FIFA president is held.
Switzerland has launched an investigation into the bidding process for the 2018 and 2022 World Cups. President Vladimir Putin's spokesman, Dmitry Peskov, said "we are assuming that Mr. Blatter will fulfil his obligations until the next congress is held, so the work is continuing and our cooperation with FIFA is continuing," in comments reported by Russia's Tass agency.
Peskov added: "The main thing is that Russia is continuing its preparations for the 2018 World Cup. All the plans are being implemented and the work is being carried out."
Russia's preparations to host the tournament are already well underway, with all of the 12 stadiums either complete or under construction.
On Tuesday, the acting president of the Russian Football Union said he was backing UEFA president Michel Platini to succeed Blatter.
South Africa's sports minister has denied that a $10 million payment was a bribe to secure the 2010 World Cup.
Fikile Mbalula told reporters Wednesday in Johannesburg that the government wanted to "categorically deny" that South Africa paid any bribes to win the right to host the tournament.
Mbalula characterized the $10 million as an "above-board payment" to help football development in the Caribbean region of former FIFA vice president Jack Warner.
Mbalula also says the South African government hadn't yet received details of the indictment from the U.S. Justice Department, which alleges the $10 million was used by South Africa to win favor from Warner and other FIFA voters to back South Africa's bid in 2004.
Sheikh Hamad bin Khalifa Al Thani, the Qatar Football Association president, says the nation welcomes the Swiss investigation into the bidding process for the 2018 and 2022 World Cups.
He was hitting back at comments from English Football Association Chairman Greg Dyke, who suggested that Qatari football organisers wouldn't be sleeping very well following Blatter's resignation announcement on Tuesday. Qatar won the bid to host the 2022 World Cup, while Russia is due to host the 2018 tournament.
"Mr. Dyke's instinct to immediately focus on stripping Qatar of the World Cup speaks volumes on his views concerning what will be the first FIFA World Cup to take place in the Middle East," Sheikh Hamad said, adding that the legal process should be allowed to take its course.
The U.S. launched a federal investigation into corruption in world football last week, issuing indictments against 14 current or former officials on May 27.
Information from The Associated Press was used in this report.