reporter: this amateur video captured the twister as it touched down. massive tornado doing massive destruction. reporter: at the heart of the destruction, st. john s regional medical center, a seven-story structure. now missing its top two floors, all of the windows have been blown out. mark harrington was in the emergency room. you heard a loud roar, all the windows started breaking. you could hear things blowing through the hospital. reporter: forecasters say this twister was one of the deadliest in missouri history and could have been worse had there been no warnings. all 25 sirens in the city of joplin did activate. according to our records, we had about a 20-minute notice when we turned on the sirens to when we actually got the first report of a tornado strike. reporter: officials are convinced advanced notice of the approaching storm saved lives. because of the stationary weather pattern holding over the midwest, the low-pressure system which is driving these nasty