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
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