The gunman who stormed the Tree of Life synagogue in Pittsburgh on Saturday, killing 11 people, was armed with an AR-15-style rifle.
It is not the first time the weapon has been used in a mass shooting in the US.
But what exactly is the weapon, why is it popular among gun owners and what other mass shootings has it been used in?
What is an AR-15 rifle?
The rifle was originally developed in the 1950s by gun-manufacturing company ArmaLite at the request of the US military, who wanted to replace the M-1 Garand and other battle rifles.
In 1959, ArmaLite sold the rights to Colt, which modified the weapon and supplied it to the US military as the fully automatic M-16.
Colt then developed a semi-automatic, civilian version — the AR-15.
The National Rifle Association (NRA) says a common misconception about the weapon is that the AR in its name stands for “assault rifle” when it actually stands for “ArmaLite Rifle”.
A key difference between the M-16 and AR-15 is that an automatic rifle continuously fires when the trigger is held down, while the user must pull and release the trigger after each shot with a semi-automatic gun.
Colt’s patent on the AR-15 expired in 1979, and numerous other manufacturers began making their own variants of the model.
But while such weapons have their own names, the popularity of the AR-15 means that the name is often used as a generic term for all AR-15-style rifles.
Why is it so popular with gun owners?
The NRA has described the AR-15 as “America’s most popular rifle”, with millions reportedly owned by people across the country.
The group has suggested that its popularity means the AR should actually stand for “America’s Rifle”.
In a 2016 blog post, the NRA said the AR-15 was popular among gun users because it is “customizable, adaptable, reliable and accurate”, and “can be used in sports shooting, hunting and self-defense situations”.
The group added that the ability to modify and personalise so many of the AR-15’s components is “one of the things that makes it so unique."
What mass shootings has it been involved in?
In the past year alone, AR-15-style rifles have been used in multiple attacks, including a school shooting in Florida in February that left 17 people dead; and the assault last October on a country music concert in Las Vegas in which 58 were killed.
Years earlier, the rifle was the focus of gun control debates when it was used in the 2012 shooting of children at Sandy Hook Elementary School in Connecticut that claimed 27 lives.
Gun control advocates say AR-15-style weapons have been used in most of the deadliest mass shootings in the US of this decade.
Are there efforts to ban the rifle?
There have been many calls to ban the AR-15 rifle over the years. Numerous people took to social media after the Pittsburgh shooting to renew calls for sales of the weapon to be restricted.
“The synagogue slaughter is another mass murder with an AR-15 assault rifle. Why are they so available?” Human Rights Watch Executive Director Kenneth Roth asked on Twitter.
“How many more mass shooting before we renew assault weapons ban?” tweeted journalist Ari Berman.
Meanwhile, supporters of the weapon say the AR-15 has been unfairly demonised, arguing that the vast majority of owners do not use it to commit a crime.
They also argue that while it has been used in numerous mass shootings, it is responsible for a small proportion of overall gun violence in the US.