FBI investigates new person of interest in Las Vegas massacre

The FBI says they believe the shooter acted alone but his motive is still unknown behind the worst mass shooting in modern U.S. history.
1:51 | 01/20/18

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Transcript for FBI investigates new person of interest in Las Vegas massacre
We turn to new developments in the massacre at the concert in Las Vegas back in October. The FBI now says it's investigating a person of interest aside from the gunman. And getting a chilling view from inside the hotel room where Stephen paddock gunned down hundreds of people killing 58. Here's ABC's Clayton Sandell. Reporter: This morning after chasing nearly 2,000 leads investigators say they still don't know the motive behind the worst mass shooting in modern U.S. History. A new sheriff's department report shows the hand da lay bay hotel room, a sniper's nest littered with high-powered rifles on countertos, chairs, beds, the floor. Other pictures capture hundreds of spent shell casings. Here a hammer with shattered glass, the broken window where Stephen paddock opened fire. The men and women of the FBI's behavioral analysis unit are working tirelessly to provide answers as to why. Reporter: The sheriff does not expect charges against paddock's girlfriend who told investigators he had become distant and withdrawn buying more guns but there is another many person of interest. There is an on going case against a person of interest. Reporter: The sheriff and FBI won't say who that is but the sheriff believes the gun Mann acted alone. Left no suicide note or manifesto and didn't appear radicalized by hate groups or terrorists. He had lost a significant amount of his monetary wealth and close proximity to 1, October, and that may have a driving factor associated with it. Reporter: The day before the gunman drove from the mandalay back to his home twice returning with six suitcases. His deadly arsenal hidden inside. For "Good morning America," Clayton Sandell, ABC news, Denver. The FBI is not saying a whole lot about that person of interest. We do want to turn to the

This transcript has been automatically generated and may not be 100% accurate.

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