Two officials told the Associated Press that two devices known as "bump-stocks" had been found along with 23 guns inside Stephen Paddock's room at the Mandalay Bay Resort and Casino.
It is not yet clear whether the stocks were fitted to the weapons that Paddock used in the massacre, but experts say they could explain the rapid rate of fire heard in videos shared on social media.
Audio analysis of one clip estimated that about 90 rounds were unleashed in only 10 seconds - far faster than a human being could repeatedly pull a trigger.
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Since Congress passed the Firearm Owners' Protection Act in 1986, it has been extremely difficult for civilians to buy new, fully automatic weapons, which reload automatically and fire continuously as long as the trigger is depressed.
It is also illegal to modify the internal components of semi-automatic rifles - which typically manage about 60 aimed shots per minute - to make them fully automatic.
However, gun owners can legally buy accessories to increase the rate of fire.
One option is a "trigger crank", "hellfire trigger", or "gat crank", which bolts on to the trigger guard of a semi-automatic rifle and depresses the trigger several times with every rotation.
A bump-stock, or "slide fire", meanwhile harnesses a rifle's recoil. It replaces the weapon's stock, which is held against the shoulder, and allows the rest of the rifle to slide back and forward with every shot despite having no mechanical parts or springs.
Following the Sandy Hook Elementary School shooting in Connecticut in 2012, California Senator Dianne Feinstein introduced a bill that sought to ban bump-stocks and similar devices, saying that manufacturers were exploiting "loopholes" to circumvent gun laws. However, the bill was defeated in the Senate.
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