Military-inspired semi-automatic rifles have been used in at least 10 mass shootings in the US since 2011.
When Omar Mateen entered an Orlando nightclub to carry out the deadliest mass shooting in US modern history, he wielded a weapon that has been used in massacres from California to Connecticut: a military-inspired semi-automatic rifle.
Though so-called assault rifles account for a small fraction of the United States' 30,000 annual gun deaths, they have been used in at least 10 mass shootings since 2011, according to a database compiled by Mother Jones magazine.
The prevalence of these firearms has made them a focal point in the debate over US gun laws as opponents say civilians should not own what they describe as "weapons of war".
Backers say they are simply modern rifles enjoyed by millions of law-abiding Americans.
In December 2012, Adam Lanza used a Bushmaster XM15 to kill 28 children and adults at the Sandy Hook elementary school in Connecticut before taking his own life with a Glock pistol.
Tashfeen Malik and Syed Farook used two assault rifles and two pistols to kill 14 people in San Bernardino, California, in December 2015.
James Holmes carried an assault rifle, a shotgun and two pistols when he killed 12 people in a Colorado movie theatre in 2012.
Law enforcement officials say Mateen, a 29-year-old US citizen who was the son of Afghan immigrants, carried an AR-15 style assault rifle and a handgun when he killed 50 people and wounded 53 at a gay nightclub in Orlando. He also had an unidentified device, said Orlando Police Chief John Mina.
The AR-15 was developed from the US military's M-16 rifle, used in the Vietnam War in the 1960s. Unlike the military version, the AR-15 is not fully automatic, meaning users must pull the trigger each time they want to fire a shot. Like the military version, many AR-15s combine light weight with a relatively modest recoil.
Prominent manufacturers include Remington, Smith & Wesson and Sturm Ruger.
The National Shooting Sports Foundation, which represents gun manufacturers, estimates there are roughly 5 million to 10 million AR-15 rifles owned in the US, a fraction of the 300 million firearms owned by Americans. Most owners say they use the rifle for target shooting and home defence, although they can be used for hunting as well.