HAWTHORNE — Hundreds of residents in a rural community steeped in military history turned out to mourn the loss of seven Marines as investigators arrived at an ammunition depot to try to determine how a mortar shell exploded at the Nevada base and sent shrapnel flying into troops during a training exercise. Families with children clutching small American flags were among the nearly 300 people who attended the brief memorial service Tuesday, where a trumpeter played taps at a city park as a giant American flag flew at half-staff across the street from the base at dusk. Marine officers from …