Lead to the gate, the perimeter wall guarded by marines, trying to separate friend from foe. >> abbey gate, follow the water. >> reporter: the last stop is the guard tower, right opposite today's blast site. inside, troops keeping watch on the crowds below. we saw one man wave his blue u.s. passport and the troops responded immediately. >> i can help you get in! abbey gate. >> you is have to wait over there. >> reporter: after the tower, it is a handful of steps out into the crowds. through the gates to the ditch that separates thousands of people from their hopes of getting inside the airport...