Las Vegas shooting: Gunman Stephen Paddock used legal 'bump stock' to let him fire so quickly for so long

The gunman who launched heavy gunfire at people at a Las Vegas concert appears to have done so using a legal trick that almost anyone can buy.

Many had wondered how Stephen Paddock managed to fire for so long and so quickly as he rained down bullets for around 15 minutes on the country music festival. The numerous weapons he fired from his hotel room allowed him to kill at least 59 people and injure hundreds more.

The speed of that shooting led some to suggest that Paddock was using automatic weapons. But automatic rifles are heavily regulated and difficult to buy in the US.

Instead, he used a "bump stock" on at least two of the weapons, officials said. That legal trick is an attachment for the weapon that technically allows it to count as semi-automatic, despite the fact that it can be used to fire like an automatic rifle.

The devices have attracted scrutiny in recent years from authorities. But they are entirely legal and regularly available in many states.

California Senator Dianne Feinstein 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 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 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.

Las Vegas shooter Stephen Paddock had 23 guns in his hotel room.

Two officials familiar with the investigation told the AP that Paddock had bump stocks attached to two semi-automatic guns. The U.S. officials were briefed by law enforcement and spoke on the condition of anonymity because the investigation is ongoing.

Paddock killed 59 people and wounded hundreds more at a country music festival near his hotel. Police stormed his 32nd floor hotel room and found that he had killed himself after committing the deadliest mass shooting in modern U.S. history.

Additional reporting by Associated Press