WASHINGTON, July 4 — The federal authorities, concerned about a terror attack during this summer's national political conventions, have begun a new effort to identify potential extremists inside the United States, including conducting interviews in communities where terrorists might seek refuge, government officials said. The fears about an incident during the conventions or later in the year have also led state and local officials to impose extraordinary security precautions. Persistent if indistinct intelligence reports, based on electronic intercepts and live sources, indicate that Al Qaeda is determined to strike in the United States some time this year, the officials said...