Investigators believe Las Vegas gunman had severe undiagnosed mental illness: Sources

Authorities are particularly focused on the period of September to October 2016.

— -- The picture that is emerging of Steven Paddock, who has been identified as the gunman in Sunday's mass shooting in Las Vegas, is of a man that some law enforcement officials increasingly believe had severe mental illness that was likely undiagnosed, sources tell ABC News.

The portrait, gleaned from interviews with hundreds of people interviewed over the past week, is that while Paddock might have been financially successful, he had real difficulty interacting with people. He is described as standoff-ish, disconnected, a man who had difficulty establishing and maintaining meaningful relationships.

The 64-year-old was known for playing gambling games in casinos for hours at a time with little or no human contact. One source told ABC News that Paddock was exhibiting many antisocial traits that are typical of past mass shooters.

ABC News has learned authorities are particularly focused on the period of September to October 2016, when Paddock began buying 30-plus guns, in concentration -- most of which were rifles.

Sources say there's evidence that his gambling wagers began increasing in scale in that time frame too.

Paddock, they say, was recently on his computer looking at a lot of different hotel venues -- some apparently just to research, some of which he actually traveled to.

According to multiple law enforcement officials, police still have found no definitive evidence to prove Paddock had an accomplice, and have not nailed down a definitive motive.

Authorities are still trying to assess whether the phone charger that doesn't fit Paddock’s phone found in the hotel room is of significance.

An image of a man that looks like Paddock, apparently accompanied by two others, is still under review and investigation. It is unclear if this image is of the gunman or if it will be meaningful.

Authorities are now beginning to wonder whether they will ever find a definitive motive and reason for the massacre, but officials are determined to keep digging. Meanwhile, FBI forensics experts and evidence technicians continue to examine numerous items flown in from Vegas to the FBI's lab in Quantico, Virginia, to exploit them for new details. One source said every technique and technology available is being used.