The Las Vegas Metropolitan Police released bodycam footage on Oct. 3 that shows the scene of the Las Vegas shooting. (Las Vegas Metropolitan Police)

Law enforcement officials said Tuesday at least a dozen of the 23 firearms recovered in Las Vegas were semiautomatic rifles legally modified to fire like automatic weapons, using an alteration known as a bump fire stock.

Stephen Paddock killed 58 people and injured hundreds more at a country music concert, as he fired from the 32nd floor of the Mandalay Bay Resort and Casino Sunday night.

Photos obtained by local Boston affiliate Fox 25 and posted Tuesday offer a glimpse into the level of preparation by Paddock to carry out different styles of attacks in a frantic nine-minute window of intermittent gunfire.

One of the two images of rifles published by the station show an AR-15 type rifle with a bump or slide fire modification, said Cody Wilson, director of Defense Distributed, which primarily sells a gun milling machine.

Police found 12 weapons with bump-fire stocks in the shooter's hotel room. These devices can be used to make semi-automatic weapons perform like machine guns. (Patrick Martin/The Washington Post)

Bump fire stocks, first made by the company Slide Fire, are legal modifications to the lower receiver of rifles that simulate automatic fire. The modified stock harnesses the energy from recoil, forcing the firing mechanism to move faster than originally designed, according to the Trace, a nonprofit website that examines gun violence in the United States. Numerous videos online, including from Slide Fire, demonstrate the mechanics.

The modified rifle with the bump fire mechanism includes a high-capacity magazine that can hold between 60 and 100 rounds, an EOTech holographic sight and a forward hand grip — a key accessory that allows a shooter to push the rifle away from the body in order to bounce, or bump, the weapon into the trigger finger.

While the attack on the Las Vegas strip is the deadliest in modern American history, attacks in the 19th and 20th centuries had higher death tolls. Here are two deadly events in American history that you may not have heard about. (Victoria Walker/The Washington Post)

Those components used together in a weapon suggest a desire to shoot large amounts of automatic-like fire into masses of people without much concern for accuracy, as the recoil from simulated automatic fire would make it difficult to hit specific targets at a long range. The stage was between 400 and 500 yards from the hotel.

“He obviously tried this out before he was in the hotel room,” Wilson said. He added his initial conclusion from audio of the gunfire pointed to a bump modification on at least one weapon. The fire rate was inconsistent with a weapon originally designed to fire automatically, Wilson said.

Another photographed weapon, however, shows a meticulous collection of accessories that indicate Paddock also readied himself for precision fire.

An AR-15 type weapon is shown mounted with a magnification scope commonly used for hunting, which helps the shooter acquire individual targets for single shots. The bipod legs at the front of the weapon would be used to steady the rifle on most hard surfaces, like a table or other furniture, which could have also provided distance away from the window to help conceal some smoke and gunfire flashes.

That weapon does not appear to be modified beyond the accessories, Wilson said.

Law enforcement officials did not immediately return requests to confirm the photos were of weapons recovered from the incident.

A third weapon, identified as an AK-47 type rifle, was outfitted with a stand to steady it and improve accuracy, said people close to the ongoing probe. Clark County Sheriff Joseph Lombardo said the weapons that have been recovered range in caliber from .223, which is associated with AR-15 style rifles, to .308, which is a caliber commonly used in hunting rifles. Lombardo was unsure if any of the weapons were originally designed as automatic, or if any illegal modifications were used on other weapons.

There are other controversial albeit legal modifications to simulate automatic fire, like an attachable crank, a gunmetal version of a jack-in-the-box handle that attaches to the weapon and strikes the trigger faster than a finger. It can be bought online for as little as $40.

Modifications like the bump fire and cranks do not technically qualify adapted rifles as automatic weapons or machine guns, which are highly regulated in the United States. Those weapons are commonly found at gun shows and firing ranges, including one near Las Vegas that allows customers to shoot military-grade weapons.

Machine guns can be legally owned if made before May 1986 and registered with the federal government, or owned by licensed dealers.

Some states, such as California, regulate the size of high-capacity magazines like the one photographed at the scene in Las Vegas, but Nevada does not. A trained shooter can easily remove one magazine from the weapon, load another from a tactical vest or pocket, and continue firing within seconds.

Stephen Paddock was identified by police as the gunman in the deadliest mass shooting in modern U.S. history. Here's what you need to know about him. (Monica Akhtar/The Washington Post)

The high number of firearms found at the hotel suggest a concern for overheating rifle barrels. The barrels can reach several hundred degrees, turning orange and even blue as rounds travel thousands of feet per second through them.

Automatic weapons used by the military have lever releases on the barrel, allowing one to be swapped for another using a oven-mitt-like glove.

The National Rifle Association says Nevada law allows the purchase of machine guns and silencers in compliance with federal law and regulations.

Devlin Barrett contributed to this report.

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