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.
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 assault rifles is hugely controversial in America as they could allow a rogue gunmen to kill dozens of people in a matter of seconds.
It was speculated on Sunday that Mateen, 27, 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.
It means Mateen is unlikely to have resorted to illegal methods to obtain his arsenal, which also included a handgun.
The emergence of yet another mass killer who easily procured an AR-15 is likely to add further fuel to America's fierce debate over gun ownership.
Piers Morgan, the British broadcaster, is among those who has called for all military-grade weapons to be banned.
"AR-15 assault rifles are banned in UK. Guess what? Nobody ever gets shot with an AR-15 in the UK. Wake up, America," he wrote on Twitter in the aftermath of the massacre.
"The #Orlando shooter was U.S. citizen with no criminal record who held TWO current gun licences. A 'good guy with a gun' until he wasn't," he added.
The National Rifle Association has defended the mass sale of assault rifles, insisting they are useful for wild hunting and "home defence."
