WASHINGTON (Reuters) - The Speaker of the U.S. House of Representatives, Dennis Hastert, is under investigation by the FBI, which is probing corruption in Congress, ABC News reported on Wednesday. ABC, citing high level Justice Department sources, said 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 those of other tribes. Hastert's Press Secretary told ABC in a statement: "We are not aware...