The Las Vegas shooter apparently turned one of his rifles into a fully automatic weapon with an inexpensive modification that can easily be bought online.

It appears Stephen Paddock -- the man who brutally murdered 59 people and injured over 500 -- attached what's called a slide fire stock or bump stock to one of his 23 guns before spraying hundreds, if not thousands, of bullets into a crowd of concertgoers from his 32nd floor Mandalay Bay hotel room.

The stock allows the shooter to hold the trigger once and fire in rapid fashion at hundreds of rounds per minute, essentially making it a fully automatic rifle or something extremely close to it.

Automatic weapons are nearly impossible to obtain but a stock runs about $160 online, and it's legal.

It appears Paddock also had a SureFire 100 magazine hooked on the rifle with the stock. The 64-year-old reportedly had multiple high-capacity magazines loaded. 

Sunday's shooting is the deadliest in modern U.S. history.