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crime before, that is something that will be key to try to get to the bottom of this. >> let's hope that leads them to the suspect. thank you very much for the latest on that investigation. joining us now is security correspondent josh campbell, tom gburny, a former detective, anda former assistant secretary at the department of homeland security. joulia, i want to start with yo. i was so interested that n something that you said earlier on the air. you said in all of your training for a hypothetical mass transit attack like this, the first thing you do is shut down the subway system. they didn't do that here. but i don't know what that would look like. how do you shut down mass transit system at rush hour that 8 million people rely on. >> it would have been subway station specific. what you don't want is the ability -- you don't want trains coming in because you don't know if there's an ongoing scene. you want to stop the trains coming in.