Image copyright Getty Images Image caption Bump-stocks can be fitted to standard semi-automatic rifles like the Colt AR-15

Police say that the gunman who killed almost 60 people at a concert in Las Vegas had outfitted a legal but controversial accessory onto 12 of his semi-automatic rifles to enable them to fire hundreds of rounds per minute.

Officials say that theses devices - known as bump-stocks - have been found along with 23 guns inside Stephen Paddock's room at the Mandalay Bay Resort and Casino.

Bump-stocks, or slide fire adapters, allow semi-automatic rifles to fire at a high rate, similar to a machine gun, but can be obtained without the extensive background checks required of purchasing automatic weapons.

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.

Lawmakers have questioned the legality of these devices while gun owners - sensing a legislative crackdown - have reportedly begun stockpiling them.

Image copyright @Boston25/Twitter Image caption One of the rifles recovered from the crime scene appears to be fitted with a bump stock

Since Congress passed the Firearm Owners' Protection Act in 1986, it has been relatively difficult for civilians to buy new, fully automatic weapons, which reload automatically and fire continuously as long as the trigger is depressed.

However, thousands of "grandfathered" weapons - those manufactured and registered before 1986 - can still be bought but are very expensive and all sales must be approved by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF).

Media playback is unsupported on your device Media caption How the horror unfolded - in two minutes

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.

Gun owners can instead legally buy accessories to increase the rate of fire.

One option is a "trigger crank", "hellfire trigger", or "gat crank", which bolts onto the trigger guard of a semi-automatic rifle and depresses the trigger several times with every rotation.

But the bump-stock, which was used by Stephen Paddock in Sunday's Las Vegas shooting, 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.

Media playback is unsupported on your device Media caption What gunfire tells us about weapons used

The motion makes the trigger collide with, or bump, the shooter's finger as long as they apply forward pressure with the non-shooting hand and rearward pressure with the shooting hand.

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.

In the wake of the Las Vegas shooting, Mrs Feinstein re-introduced a bill on Wednesday that would outlaw the sale and possession of bump-stock equipment and other similar devices.