US Intelligence warns of possible threats to large gatherings

U.S. law enforcement officials are concerned radicals in the U.S. might respond to ISIS calls for similar attacks in the wake of what happened in Russia, according to a bulletin obtained by ABC News. Subscribe to ABC News on YouTube: Watch 24/7 coverage of breaking news and live events on ABC News Live: Watch full episodes of World News Tonight with David Muir here: Read ABC News reports online: ABC News Digital is your daily source of breaking national and world news, exclusive interviews and 24/7 live streaming coverage. ABC News is the home to the #1 evening newscast “World News Tonight” with David Muir, “Good Morning America,” “20/20,” “Nightline,” “This Week” with George Stephanopoulos, “ABC News Live Prime” with Linsey Davis, plus the daily news podcast “Sta