Orlando Gunman Used a Kind of Assault Rifle Popular Among Mass Shooters

— -- The man suspected of killing at least 49 people at a gay nightclub in Orlando apparently used a kind of semiautomatic assault rifle that has become a weapon of choice among mass shooters.

He had a .223 caliber AR-type rifle and a Glock handgun on him at the time of the shooting early this morning, law enforcement sources told ABC News.

The AR-15 was used by the couple in San Bernardino, California, who killed 14 people at a workplace holiday party in December. Similarly, the man who mowed down 12 people at a movie theater in Aurora, Colorado, in 2012 was armed with an AR-15.

And the man who killed 20 first-grade children and six school staff members at an elementary school in Newton, Connecticut, in 2012 used the same kind of weapon.

The AR-15 was designed for military use in the 1950s and has since become popular among civilian gun owners. And, increasingly, it is becoming notorious for its use in massacres.

Richard Frankel, an ABC News contributor and former FBI special agent in charge of counterterrorism in New York, said the AR-15 may be attractive to mass shooters because it doesn’t require much training to use and is easy to get.

“It’s very easy to operate,” he said. “It’s easy to load, point and shoot.”

The gun is sold by many U.S. gun retailers, he added.

Frankel said it's possible that some who are intent on killing a lot of people copy using other mass shooters’ weapons.

The AR-15 can easily hold 30 rounds at a time, he said. It shoots "as fast as your finger goes," he said.

A shooter could easily fire 30 rounds in less than a minute and then quickly reload to fire more, he said.

The capacity of the rifle to inflict maximum damage in minimal time is part of the basis of a lawsuit by families of victims in the Newtown shooting against the manufacturer of the gun used in that incident.

"There is one civilian activity in which the AR-15 reigns supreme: mass shootings," the parents allege in their complaint filed in October in a Connecticut court. "Time and again, mentally unstable individuals and criminals have acquired an AR-15 with ease, and they have unleashed the rifle's lethal power into our streets, our malls, our places of worship and our schools."

Remington Outdoor Co., the parent company of the manufacturer of the AR-15, has declined to comment to ABC News, citing the pending litigation, but did cite a 2005 federal law in which gun manufacturers and dealers are shielded from liability after mass shootings.

The National Rifle Association contends that the AR-15 is popular among ordinary, law-abiding gun owners because it is "customizable, adaptable, reliable and accurate [and] can be used in sport shooting, hunting and self-defense situations," according to an NRA blog entry in January 2016.