The FBI has warned local authorities of a threat that Islamic State militants would blow up the Memphis & Arkansas Bridge spanning the Mississippi River, an agency spokesman said on Tuesday. The bridge is a major route connecting Tennessee and Arkansas. The Federal Bureau of Investigation passed the threat on to local police in a bulletin out of an "abundance of caution," spokesman Chris Allen said. "This is an unsubstantiated, anonymous threat," he said, adding that there was no useful intelligence arising from it.