The first question, however — as to what persuaded Mr. Blatter to leave — is more complex. While Mr. Blatter has not been directly implicated in any criminal cases, the disclosure that law enforcement officials in the United States are targeting him speaks to the legal vulnerability he may be facing.

A high-ranking soccer official said Mr. Blatter had been advised by his legal counsel that continuing in his current position could make defending him against possible future prosecution more difficult.

The soccer official, speaking on the condition of anonymity because he was not authorized to discuss the matter publicly, said in particular there were concerns about Mr. Blatter, as president, needing to make public comments on either the continuing Justice Department investigation or a separate investigation by Swiss authorities into allegations of improprieties during the awarding of the hosting rights for the 2018 and 2022 World Cup tournaments. While such comments might be considered the norm for an organization’s leader, the official said, in this instance they could create additional legal problems for Mr. Blatter.

The soccer official added that pressure on Mr. Blatter from soccer’s corporate partners, as well as from various FIFA members, increased considerably over the weekend as it became clear that the Justice Department indictment was not looking just at corruption within Concacaf, the regional governing body overseeing soccer in North America, Central America and the Caribbean.

Initially, much of the public focus of the investigation was on Concacaf’s role, and FIFA officials, including Blatter, portrayed the arrested officials as rogues. But The New York Times on Monday, based on information from several United States officials and others briefed on the case, linked Mr. Blatter’s top deputy, Jérôme Valcke, to a series of payments that are believed to be bribes connected to South Africa’s winning the vote that gave it the 2010 World Cup.