Clark County Sheriff Joe Lombardo discusses the Route 91 Harvest festival mass shooting at the Las Vegas Metropolitan Police Department headquarters in Las Vegas, Monday, Oct. 9, 2017. Law enforcement authorities on Monday made a significant change to the timeline of the mass shooting, saying the gunman shot a hotel security guard before he opened fire on concertgoers. (Erik Verduzco/Las Vegas Review-Journal via AP)
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More than a week after the deadliest mass shooting in modern U.S. history investigators are stumped about the key question: What led a 64-year-old high-stakes gambler to kill 58 people and wound hundreds of others at a country music concert?