A gay nightclub in Orlando, Florida was the scene early Sunday of the worst terror attack in U.S. history since 9/11. The shooting happened at Pulse at around 2 a.m., just before closing time. According to law enforcement, the gunman was identified as U.S.-born citizen Omar Mateen of Ft. Pierce, Florida, and was eventually shot and killed by Orlando police.

Omar Mateen, Gunman of 2016 Orlando Mass Shooting.

Officials, aided by the FBI, warned that a lengthy investigation was ahead, given the number of victims and the scope of the violence.

Outside Pulse Nightclub, scene of massive Orlando shooting.

According to a 2013 study, there are roughly 357 million firearms in the U.S. That means that there are roughly 40 million more guns than there are people in America.

But what is the connection between one American mass murder to the next?

This weapon.

AR-15, an easily obtainable weapon in the U.S.

The AR-15 is the most common military-grade weapon in the United States. It has long been favored by police for its accuracy, low recoil and dependability. It has a magazine capacity of 30 rounds. Just four of those is 120 rounds of ammunition, which can be reloaded in a matter of a second.

The weapon is also the most common American long rifle; it's easy to find and costs about $500 to $700. You can buy ammunition at Walmart for 25 cents a bullet.

Shooting 50 to 100 people is just a matter of pulling a trigger.

The fact of the matter is... it doesn't take a genius to manipulate this weapon.

According to MSNBC and analyst Malcolm Nance, "The entire design of this attack is to get past the front door, past the security, and then get into a confined space where they can carry out their mass murder. We had this discussion eight weeks ago during the attacks in Brussels, where I was asked what is the nightmare scenario for the United States by a radicalized terrorist. The scenario for me was a confined space, mass murder using legally acquired weapons. And it appears to be where we are."

Students line up to escape Sandy Hook massacre, 2012.

The same military-grade weapon was used by Adam Lanza in December 2012 to shoot dead 26 people at Sandy Hook in Newtown, Connecticut, by James Holmes the same year to kill 12 people in a movie theatre in Aurora, Colorado, by the San Bernardino terrorists when they killed 14 people last December, by Chris Harper-Mercer when killing nine people at Umpqua Community College in Oregon.

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Aurora victim's mother on Twitter.

Fear is striking the hearts of millions. We live in a country where it is easy to obtain these weapons legally, and it is simple to carry out such horrendous acts of terrorism. Anywhere, at any time, in movie theaters, malls, clubs, high schools, elementary schools, universities... we continue to live in fear of a massive attack.

How do we fix this?

Piers Morgan, British broadcaster, is among those who has called for all military-grade weapons to be banned.

Among gun rights advocates, the AR-15 actually appears to be popular. Michele Bachmann, American politician and former member of the House of Representatives, spoke highly of it.

“My favorite gun is an AR-15 because you can be so accurate with it. And I scored the best in my class of any of the men too.”

When asked by her interviewer, “And that’s a combat weapon?” the then-presidential candidate responded, “I love it. It’s a great gun.”

Amidst the blood bath, the gun control debate continues.

Shocking photos from the Aurora, Colorado movie theater crime scene, 2012.