National Transportation Safety Board investigators are trying Friday to determine what caused part of an Interstate 5 bridge to fall into the Skagit River 55 miles north of Seattle on Thursday. Just before the collapse, a semitruck carrying a heavy load struck the side of the four-lane bridge. The collapse sent two cars plunging into the frigid water. Nobody was killed, but three people were rescued from the river and taken to hospitals. Two had hypothermia.