PHOENIX — About 10 hours after the massacre that claimed the lives of his grandchildren, Kenneth Miller drove his ATV through the northern Mexico wilderness in a frantic search for one last missing relative — a 9-year-old girl who had gone in search of help.

Following a path off the dirt roadway where three vehicles had been hit with a barrage of gunfire, Mr. Miller and fellow searchers found small footprints in the sand — one foot bare, the other with a shoe on it. They followed the tracks for miles, at times losing the trail on harder rock, then picking it back up again in softer soil.

Then, through the darkness, Mr. Miller saw her. Mckenzie was alive.

“I ran out and grabbed that little girl and just hugged her,” Mr. Miller recalled. “I said, ‘It’s your Uncle Kenny.’ The first thing she said was, ‘We’ve got to go back and get the others.’”