The Speaker of the House of Representatives, Dennis Hastert, is under investigation by the FBI, which is seeking to determine his role in an ongoing public corruption probe into members of Congress, ABC News has learned from high level official sources. Federal officials say the information implicating Hastert was developed from convicted lobbyists who are now cooperating with the government. Part of the investigation involves a letter Hastert wrote three years ago, urging the Secretary of the Interior to block a casino on an Indian reservation that would have competed with other tribes. The other tribes were represented by convicted...