This undated photo provided by Eric Paddock shows his brother, Las Vegas gunman Stephen Paddock.

:: The gunman who unleashed hundreds of rounds of gunfire on a crowd of concertgoers in Las Vegas had two “bump-stocks” that could have converted semi-automatic firearms into fully automatic ones, officials said.

The devices have attracted scrutiny in recent years from authorities. California Sen. 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.

Support step

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

A sign reading “Blood Donations Needed” is seen in Las Vegas Boulevard on October 2, 2017.

Mourners attend a candlelight vigil at the corner of Sahara Avenue and Las Vegas Boulevard for the victims of Sunday night’s mass shooting, on October 2, 2017 in Las Vegas, Nevada.