 They have gone to some of the biggest crime scenes really around the world, but especially in the U.S., often going to help other governments, experts in what they do. You're listening to live coverage from ABC News. It was now a number of hours ago when shots were heard. At the moment, it sounded like popping noises. Some people thought they were fireworks. Others knew immediately it was gunfire unfolding at former President Trump's rally in Butler, Pennsylvania, north of Pittsburgh.