The Las Vegas gunman used "bump stocks'' to turn at least a dozen rifles into something akin to automatic weapons, as well as remote video cameras to alert him when police were closing in on his firing positions, according to law enforcement officials.
One of the cameras was placed on a room service food cart just outside Stephen Paddock's hotel room door, in what officials said was an apparent effort to get an early warning when police were about to break in. At least one other camera was positioned inside the room, apparently for the same reason, officials said.
People familiar with the investigation said the cameras were linked to a tablet device that allowed the gunman to "watch his back" for first responders while firing down on the concert crowd from his 32nd-story suite, killing 58 people and injuring more than 500 others.
The cameras and weaponry speak to what officials said Tuesday was a remarkable amount of planning and preparation by the shooter to increase the lethality of his attack. He may have used the cameras to ensure he was not captured alive, according to people familiar with the investigation.
When a SWAT team burst into his room, they found him dead on the floor, apparently having shot himself in the mouth with a silver, black-handled revolver.
Officials revealed more details of the 23-gun arsenal Paddock brought up to Room 32-135 of the Mandalay Bay Resort and Casino. He hauled them in suitcases and apparently was stealthy enough that hotel staff members never spotted the weapons and stacks of ammunition magazines.
Detectives believe he used a hammer to break out two windows in the large suite, then moved between the openings, alternating weapons as he alternated windows, according to people familiar with the investigation.
After the attack, investigators found another 19 guns at the shooter's home in Mesquite, Nev. Authorities say the weapons purchases appear to have been legal, some acquired at local stores near his home and others bought at chain retailers such as Cabela's.
ATF officials said Tuesday evening that 12 of the guns recovered in Las Vegas had "bump fire'' stocks — devices which make semi-automatic weapons fire almost as fast as fully automatic weapons. Investigators are still working to determine how many guns in his arsenal he used.
Agents from the FBI's headquarters were in Las Vegas to help collect evidence, some of which would be sent back to FBI labs for further testing and evaluation, Clark County Sheriff Joseph Lombardo said.
The arsenal included AR-15-type rifles, as well as an AK-47-type weapon, according to officials. Some of the weapons were mounted on bipods. At least one had a scope.
Photos obtained by local Boston affiliate Fox 25 show an AR-15-type rifle on the floor of the hotel room. The weapon has a bump- or slide-fire modification, said Cody Wilson, director of Defense Distributed, which primarily sells a gunmaking machine.
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 Bureau of Alcohol, Tobacco, Firearms and Explosives decided in 2010 that bump stocks do not violate federal firearms laws.
The modified rifle with the bump-fire mechanism included a high-capacity magazine that could hold between 60 and 100 rounds, an EOTech-brand holographic sight and a forward hand grip — a key accessory that allows a shooter to push the rifle away from the body to bounce, or bump, the weapon into the trigger finger.
Those components used together in a weapon suggest a desire to quickly shoot a lot 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 long range. The concert 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 that 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 indicating that the shooter also readied himself for precision fire.
An AR-15-type weapon is shown mounted with a magnification scope commonly used for hunting, which assists in focusing on individual targets for single shots. The bipod legs at the front of the weapon would have been used to steady the rifle on hard surfaces, such as a table, a move that could have provided distance from the window that would help conceal smoke and gunfire flashes.
That weapon did not appear to have been modified beyond the accessories, Wilson said.
Las Vegas officials have opened an internal investigation into how the photos of the crime scene became public.
Lombardo said authorities have recovered .223-caliber weapons — associated with AR-15-style rifles — as well as at least one .308 rifle — a caliber common for hunting game. Lombardo was unsure whether any of the weapons were originally designed as automatic, or whether any illegal modifications were used on other weapons.
Other controversial yet legal modifications can simulate automatic fire, such as attachable cranks, gunmetal versions of a jack-in-the-box handle that attach to the weapon and strike the trigger faster than a finger can. A crank can be bought online for as little as $40.
Modifications such as the bump stock and crank do not technically qualify adapted rifles as automatic weapons or machine guns, which are legal but difficult to obtain and 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.
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. In one crime-scene photo published by the Daily Mail, the gunman appears to be wearing a glove on his left hand, probably for holding a smoking-hot barrel.