Paddock is believed to have removed the hard drive before fatally shooting himself, and the missing device has not yet been recovered, sources told ABC News.

Investigators digging into Paddock's background also learned he purchased software designed to erase files from a hard drive, but without the hard drive to examine it is impossible to know if he ever used the software, one source said.

The absence of substantial digital clues has left investigators struggling to piece together what triggered Paddock to kill 58 innocent concertgoers and injure more than 500 others on Oct. 1.

Authorities are examining every aspect of Paddock's life — from his family, friends and associates to his travel patterns, health and finances. So far, a motive has been elusive to investigators.