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Officials: Las Vegas shooter had device allowing gun to fire like an automatic

What is a "bump stock"?
What is a "bump stock"? 02:10

LAS VEGAS — The man who unleashed hundreds of rounds of gunfire on a crowd of concertgoers in Las Vegas had at least one accessory that could have allowed his semi-automatic rifles to fire rapidly and continuously, as if they were fully automatic weapons, officials said. 

Though legally and widely available, the so-called "bump stocks" have attracted scrutiny from authorities and lawmakers in recent years. 

Sen. Dianne Feinstein, D-Calif., has long railed against them. Several years ago, she told The Associated Press she was concerned about the emergence of new technologies that could retrofit firearms to effectively make them fully automatic. 

"This replacement shoulder stock turns a semi-automatic rifle into a weapon that can fire at a rate of 400 to 800 rounds per minute," she said. 

A semi-automatic weapon requires one trigger pull for each round fired. With a fully automatic firearm, one trigger pull can unleash continuous rounds until the magazine is empty. The bump-stock devices work by manipulating the trigger mechanism extremely rapidly, far faster than a person could do so without them.

Tracking down the gunman 01:30

Clark County Sheriff Joe Lombardo confirmed Tuesday that a bump stock was found in gunman Stephen Paddock's hotel room. Two officials familiar with the investigation told AP earlier Tuesday that Paddock had two of the devices.

They are investigating whether those items were used to modify weapons used in the massacre, according to those officials, who were briefed by law enforcement and spoke on condition of anonymity because the investigation is still unfolding. 

Lombardo on Tuesday did not specify whether one or more of the bump-stock devices were found. 

On Monday, Lombardo said investigators believed Paddock had modified weapons, but did not specify how. Tuesday, he again wouldn't comment on how the weapons were modified, saying the ATF is investigating.

The ATF has said it is conducting an "urgent trace" on the firearms.

Police said Paddock had at least 23 firearms in his hotel suite. He had been staying in the room since Sept. 28. They said another 19 firearms, thousands of rounds of ammunition, explosives and electronics were found at his home at a retirement community in Mesquite, Nevada, about 75 miles northeast of Las Vegas. Lombardo said Monday investigators believed Paddock used multiple weapons in the massacre, but couldn't say how many.

Authorities say Paddock opened fire from the windows of his 32nd floor hotel room late Sunday, killing 58 people and wounding hundreds more at a country music festival. Police stormed his room and found he had killed himself after committing the deadliest mass shooting in modern U.S. history. 

Witnesses and law enforcement official said the quick, 50-round bursts of gunfire raised the possibility that Paddock had used a fully automatic weapon or modified his semi-automatic rifles to function like one. 

Yet the purchasing of fully automatic weapons has been significantly restricted in the U.S. since the 1930s. 

In 1986, the federal National Firearms Act was amended further to prohibit the transfer or possession of machine guns by civilians, with an exception for those previously manufactured and registered. 

Numerous attempts to design retrofits failed until recent years when bump stocks came on the market. 

The device basically replaces the gun's shoulder rest, with a "support step" that covers the trigger opening. By holding the pistol grip with one hand and pushing forward on the barrel with the other, the shooter's finger comes in contact with the trigger. The recoil causes the gun to buck back and forth, "bumping" the trigger. 

Technically, that means the finger is pulling the trigger for each round fired, keeping the weapon a legal semi-automatic. 

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