What is an AR-15, was the rifle used in the Pittsburgh synagogue attack and what other mass shootings has it been used in?
THOMAS Bowers used an AR-15 to kill 11 worshippers at the Tree of Life Synagogue in Pittsburgh, the authorities say.
The military-style weapon can discharge dozens of rounds in a minute and has been used in several other mass shootings - here's what you need to know about it.
What is an AR-15?
The weapon is a lightweight semi-automatic rifle, described by the National Rifle Association as the most popular rifle in America.
Over the decades many different types of this rifle have been made.
It was originally designed for troops to use in conflict and was easily adapted for civilian use, but has one small difference.
The military weapons can fire fully automatically, however the civilian version requires a pull of the trigger for each shot.
It is fed with box magazines - that hold 30 rounds - that can be quickly changed.
Someone wielding this weapon could fire more than a hundred rounds in minutes.
How did Thomas Bowers and Nikolas Cruz get the rifle?
Police said Bowers used an AR-15 and three Glock .357 handguns during the attack.
In 2017, Pennsylvania became the last state to legalise semi-automatic rifles, including AR-15s, for hunting.
To buy a gun in Pennsylvania you must be 18 though there are restrictions such has having a conviction for a violent crime or being declared mentally ill by a court.
According to and involves going into a licensed gun store.
Customers then apply for a background check which costs £17.50 for a handgun and £22 for a larger weapon.
Pennsylvania State Police Trooper Adam Reed said; "In most cases, the check goes through within minutes - very quickly."
He legally purchased the three Glock .357s, a law enforcement
But bit it's not clear whether the AR-15 was bought in that way.
Nicolas Cruz, who has appeared in court charged with the murder of 17 people at the Marjory Stoneman High School in Parkland, Florida walked into a gun store and legally bought it.
This is because in Florida 18 year olds can buy semiautomatic rifles.
There is a three-day waiting period for handguns, but anyone over 18 can step into a shop, have a brief background check and walk out carrying a rifle.
Nikolas Cruz, 19, was able to legally buy the AR-15 rifle and keep it in a lockbox in his bedroom, before taking it to the school on February 14.
No laws were violated in the purchase of the weapon used to pepper schoolkids and teachers with bullets.
He confessed to investigators he carried out the deadly shooting, adding he had brought a backpack full of ammunition to the massacre.
Pictures of guns and knives had been previously posted to an account, now deleted, on Instagram thought to have been run by him.
And a YouTuber, thought to be him, posted a comment saying "I want to be a professional school shooter", which was investigated by the FBI.
In a country where gun laws are the subject of huge debate from both sides, AR-15s and other semiautomatic rifles had been banned by Congress from 1994-2004, but this law expired 14 years ago.
Now in most states it is easy to buy a rifle like this one - with some private sellers online and in person not required to perform a background check in "the gun show loophole".
Only New York, California, Washington, D.C., and five other states have assault-weapon bans.
A post on the NRA website states American's own about five million Ar-15s.
What other mass shootings has an AR-15 - or similar rifle - been used in?
- February 24, 1984: Tyrone Mitchell, 28, killed two and wounded 12 at a Los Angeles school before killing himself
- October 7, 2007: Tyler Peterson, 20, killed six and injured one at an apartment in Crandon, Wisconsin, before killing himself
- June 20, 2012: James Eagan Holmes, 24, killed 12 and injured 58 at a cinema in Aurora, Colorado
- December 14, 2012: Adam Lanza, 20, killed 27 people in Newtown Connecticut, before killing himself
- June 7, 2013: John Zawahri, 23, killed five and injured three in Santa Monica, California, before he was killed
- March 19, 2015: Justin Fowler, 24, killed one and injured two on a street in Little Water, New Mexico, before he was killed
- May 31, 2015: Jeffrey Scott Pitts, 36, killed two and injured two at a store in Conyers, Georgia, before he was killed
- October 31, 2015: Noah Jacob Harpham, 33, killed three on a street in Colorado Springs, Colorado, before he was killed
- December 2, 2015: Syed Rizwyan Farook and Tashfeen Malik, 28 and 27, killed 14 and injured 21 in California, before they were killed
- June 12, 2016: Omar Mateen, 29, killed 49 and injured 50 at an Orlando nightclub, before he was killed
- October 1, 2017: Stephen Paddock, 64, killed 58 and injured hundreds at a music festival in Las Vegas, before he killed himself
- November 5, 2017: Devin Kelley, 26, killed 26 at a church in Sutherland Springs, Texas, before he was killed