Local and federal investigators still have not come up with a motive that sparked a Nevada man to commit one of the deadliest mass shootings in American history. More than three weeks after Stephen Paddock opened fire and killed 58 people and wounded hundreds of others attending a country music festival below his Las Vegas hotel room, authorities appear stumped about uncovering a critical piece of information—Paddock's hard drive—that could potentially lead them to other suspects.

Some perpetrators of mass violence leave behind manifestos of sorts, like the one from Theodore Kaczynski, the Unabomber. His 35,000-word manifesto railing against technology paved the way for his 1996 arrest after his brother, David, realized it was written by his sibling. Paddock, who killed himself in his Mandalay Bay hotel room after the October 1 shooting rampage, hasn't left any hint of a motive to explain his murders.

The FBI is currently examining computers and cell phones in the FBI's lab in Quantico tied to the Paddock case. However, a hard drive in a laptop found in the shooter's hotel room is now missing, according to the Associated Press.

Investigators suspect Paddock removed the hard drive from the laptop after he opened fire from his high-rise suite at Mandalay Bay into a crowd at a country music festival, killing 58 people and wounded hundreds of others, the official said.

The AP added:

The hard drive hasn’t been found, according to the official, and the absence of digital clues adds yet another puzzling aspect to the investigation as authorities try to figure out what might have led the 64-year-old high-stakes gambler to commit the mass killing.

Adding to the intrigue, ABC News says that Paddock had purchased software "designed to erase files from a hard drive." The authorities did not name the software and are not sure whether it was used.

The authorities said they are also examining everything they can about the life of Paddock, his friends, associates, finances, travel patterns, health, and even his family in search of clues. Paddock's brother, Bruce, was arrested Wednesday in Los Angeles on suspicion of child porn possession. But still, so far there have been no clues about a murder motive according to the authorities.

A new normal

Absent manifestos or written notes, searches of smart phones, reviews of computer search history, and hard-drive forensics have become the norm for crime solving in modern times. Computer forensics has become so important to solving cases that the FBI demanded that Apple create a backdoor so the feds could unlock the iPhone used by one of two shooters that killed 14 people in San Bernardino in December 2015. The authorities subsequently dropped their legal battle after announcing a private firm had unlocked the phone for them.

Paddock's missing hard drive, meanwhile, is a baffling mystery in and of itself. But it's also a blow to the investigation because a digital trail can often lead to other suspects.

Consider the benefits derived from the digital investigation into Omar Mateen, responsible for last year's Orlando nightclub massacre. Although he killed himself, Mateen's digital life helped pave the way for charges to be levied against Noor Salman, his widow. The authorities allege she knew of the plot to open fire in the Pulse club, where 49 people were killed and 58 others were injured.

In the Las Vegas case, with nowhere else to turn, the authorities have now resorted to the unusual practice of erecting billboards listing an FBI hotline asking the public for help.

One billboard says: "If You Know Something, Say Something."