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 U.S. since the 1930s.