trains across the city in a calculated act of terrorism. donna friesen covers the story as a correspondent for nbc news. >> these are not suicide bombers. they are backpacks placed inside the train by terrorists who placed them and then got off the train and left them. there were virtually 25 pounds of explosives in each backpack filled with nails and screws and various shrapnel so there would be maximum damage when they exploded. they were all detonated by cell phones, so they all had cell phones in them and the perpetrators then dialed the numbers of the cell phones. >> marin is standing in the same train car holding one of the backpacks. shortly after one of the explosions knocked him unconscious, he began to come to his senses. >> translator: i started to feel the pain of not being able to move. i was paralyzed.