CHICAGO - Residents of Midwestern states mourned their dead Sunday after a string of killer tornadoes tore through the US heartland, killing at least 38 people, injuring hundreds and virtually wiping out entire communities. Church services were to be held throughout the stricken region as stunned Americans grappled with the magnitude of the destruction brought by Friday’s twisters. More trials, however, were in store for the disaster area. While tornado watches were discontinued, the National Weather Service forecast a cold front with possible snow.