AP
An old photograph of gunman Stephen Paddock, pictured next to his brother Eric, which the family provided to the media.
Stephen Craig Paddock, the 64-year-old retiree who opened fire from the window of his 32nd-story Mandalay Bay Resort and Casino hotel room on the Route 91 Harvest Festival, possessed 23 firearms and thousands of rounds of ammunition that he had procured legally.
In Nevada, where Paddock lived, anyone over 18 years of age can own a firearm.
Nevada does not license gun owners, require guns to be registered, limit the number of guns a person can purchase in one sale, or impose a waiting period on gun purchases.
Nevada allows the sale of high capacity magazines, high-caliber weapons and ammunition, and military-style weapons, though they are prohibited in other states.
But one of Nevada's few restrictions is on the sale or manufacture of automatic weapons, and the Associated Press reports that Paddock used a perfectly legal device to effectively circumvent that.
The device, known as a "bump stock," replaces the shoulder rest of a rifle with a device that bounces the weapon back into the shooter's trigger finger. Effectively, the weapon still fires one bullet for every pull of the trigger, but the bump stock automates the trigger pull process allowing shooters to fire 400-800 rounds per minute.
Gun owners can achieve a similar, though less controlled effect by sticking their finger through their belt loop or simply holding a stick between the trigger and the trigger guard.
Instead of a solid stock seen on the gun pictured above, a bump stock bounces back against the shooter's finger, automating the trigger-pulling process.
Additionally, simple but highly illegal after-market modifications can change semi-automatic, or one trigger pull, one shot weapons into automatic weapons.
Paddock rattled off hundreds of shots over a five minute period. The bullets peppered a crowd of 22,000 leaving more than 500 injured and 59 dead.
Audio from the attack clearly portrays extended bursts of rapid-fire, uniform fire that an unaltered semi-automatic weapon simply couldn't replicate while firing accurately.