A hearse containing Las Vegas shooting victim Christopher Roybal arrives at McCarran International Airport, Monday, Oct. 9, 2017, in Las Vegas, for a short flight to Southern California. His friend and employer, David Harman, told The Associated Press that Roybal served in Afghanistan and adopted a friend's bomb-sniffing dog after the friend was killed by an improvised explosive device. (AP Photo/John Locher)
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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?