Las Vegas shooter's laptop missing its hard drive

— -- A laptop computer recovered from the Las Vegas hotel room where Stephen Paddock launched the deadliest mass shooting in U.S. history was missing its hard drive, depriving investigators of a potential key source of information on why he killed and maimed so many people, ABC News has learned.

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.

The frequent gambler, described as a loner, spent months amassing his arsenal, staying under the radar with no one suspecting he would turn to extreme violence.

His finances did not appear to be a problem and Paddock had settled any debts he had with casinos shortly before the attack, a source said.

Other mass shooters in the United States have left behind writings, manifestos or even videos explaining their actions — however deranged those explanations may have been.

Paddock, however, left behind no such writings or evidence.

He is not the first mass shooter to destroy or hide digital clues.

In 2007, Virginia Tech shooter Cho Seung Hui left a video explaining his grievances, but took the step of removing the hard drive of his computer and disposing of his cell phone shortly before the massacre. Authorities even searched a pond for the missing digital media, but the devices were never recovered.

The 2008 Northern Illinois shooter, Steven Kazmierczak, removed the SIM card from his phone and the hard drive from his laptop, and neither was recovered.

In 2012, Sandy Hook shooter Adam Lanza had removed the hard drive from his computer and smashed it with a hammer or screwdriver.