Washington, June 12 

 Gunman Omar Mateen was armed with an AR-15-type assault rifle, which has become the signature weapon of mass killers in the US.

Known as "America's gun," it is owned by around 3.7 million American households, making it the country's most popular rifle, reported the Telegraph (UK) online.

The military-grade weapon was used by Adam Lanza in December 2012 to shoot dead 26 people at Sandy Hook in Newtown, while the San Bernardino terrorists used AR-15s when they killed 14 people in December 2015.

It was also used to kill nine people at Umpqua Community College in October 2015.

And in July 2012 James Eagan Holmes used an AR-15-type assault rifle to kill 12 people in Aurora, wounding a further seventy.

The sale of semiautomatic weapons is hugely controversial in America as the rapid fire mode would allow a rogue gunmen to kill dozens of people in a matter of seconds.

It was speculated on Sunday that Mateen, 29, would only have needed to reload once during his killing spree due to the weapon's large clip size. 

In the state of Florida, anyone over the age of 18 can buy an AR-15 as no state permit is required. The same goes for pistols and shotguns.