The gunman who unleashed hundreds of rounds of gunfire on a crowd of concertgoers in Las Vegas attached what is called a "bump-stock" to two of his weapons, in effect converting semiautomatic firearms into fully automatic ones.
A US law enforcement official told Reuters authorities believed Stephen Paddock, 64, had at least two such weapons when he fired from his 32nd-floor hotel room, killing at least 59 people and injuring hundreds more.
The devices have attracted scrutiny in recent years from authorities.
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 US 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.
An additional 19 were found at his home.
Two officials familiar with the investigation told the AP that Paddock had bump stocks attached to two semi-automatic guns. The US 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 US history.