The weapon used in the worst mass shooting in modern U.S. history was designed to meet U.S. military requirements for a small, quiet combat rifle for use in close quarters.

The Sig Sauer MCX semi-automatic rifle hit the market in 2015 after years of development by the Newington, New Hampshire-based company, which markets it as an "innovative weapon system built around a battle-proven core."

"The MCX was built from the ground up ... to meet all the user benefits we needed to have for military, law enforcement," Cory Newman, a Sig Sauer design engineer, says in a company promotional video.

Investigators say Omar Mateen was armed with a Sig Sauer MCX and a pistol during the attack early Sunday at Pulse nightclub in Orlando, which left 49 people dead. Mateen, who had purchased the guns legally, was killed after a shootout with police.

Gun control advocates say the MCX, which comes with a magazine that carries 30 rounds, is exactly the type of assault weapon that should not be available to civilians. Its light weight and ability to be fired and reloaded quickly make it an ideal gun to carry out an attack in a crowded place, they say.

"This is a flagrant and grotesque example of how weapons designed for the battlefield are sold to the civilian population in the United States," said Josh Sugarmann, executive director of the Violence Policy Center, which issued a report on the weapon Thursday. "The reason that shooters use weapons like this is because this is exactly what they were designed for."


He noted the gun features a folding collapsible stock that can make it smaller and a forward handgrip that allows users to repeatedly fire without burning their hands.

A message left Thursday for Sig Sauer seeking comment was not immediately returned. The National Shooting Sports Foundation, the trade association for the firearms industry, and the National Rifle Association also did not immediately return calls seeking comment.

After the Orlando shooting, gun control advocates have called on Congress to reinstate a federal ban on assault weapons that expired in 2004 and prohibit the use of magazines that carry more than 10 rounds. Critics, including the NRA, say those measures would do little to stop criminals while limiting the availability of rifles that are popular for shooting, hunting and self-defense.

Sig Sauer designed the MCX after the military expressed interest in recent years in acquiring a smaller, lighter rifle for U.S. special forces and other soldiers to use.

"The Sig MCX was designed to meet a (Department of Defense) requirement for a weapon that would be as compact as possible, that focused on signature reduction, wanted something as quiet as possible and would also engage targets out to 300 meters," Kevin Brittingham, then-president of the company's silencer division, says in a company promotional video for the weapon.

The Army awarded the first contract earlier this year under its Compact Semi-Automatic Sniper System program to another manufacturer. Sig Sauer's proposal had been the MCX M-R, which has some different features than the rifle used by Mateen.

The MCX rifle, which weighs about six pounds without a magazine, has a suggested retail price of $1,866.