Las Vegas shooting: What drove Stephen Paddock to kill 59 people?

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Investigators are struggling to find out what drove Stephen Paddock to kill 59 people and injure more than 500 in the deadliest mass shooting in U.S. history. New searches inside his hotel room and homes may shed some light. Police are also anxious to interview his girlfriend, 62-year-old Marilou Danley. Savannah Guthrie is in Las Vegas for TODAY.