The United States is peculiarly prone to mass killings by people armed with semi-automatic weapons. There is no good reason for ordinary citizens to own military-style weapons, and they have long posed a threat to American national security.

The mass murder of children at Sandy Hook Elementary School in Newtown, Conn., or the theater massacre in Aurora, Colo., both in 2012, were insufficient to spark a serious national legislative debate about this threat. Now that four Marines are dead at the hands of a civilian armed with such a weapon, can we discuss again a ban on those weapons, of the sort enacted in the Clinton administration?

Mohammod Youssuf Abdulazeez on Thursday rented a Mustang convertible and drove to a military recruitment facility in Chattanooga, Tenn., where eyewitnesses reported that he fired some 30 rounds from a rifle in just a matter of minutes. It sounded, one said, like a jackhammer. In other words, he used a semi-automatic weapon. He then drove seven miles to a reserve center for the Navy and Marines where he parked, propped up his assault weapon on the door of the car and opened fire, killing four Marines and wounding four others, including a policeman.

Also Thursday, a jury found James Egan Holmes guilty of first-degree murder for opening fire in a movie theater in Aurora, Colo. Holmes, a brilliant student, had long suffered from mental illness and was considered dangerous by his psychiatrist. On June 7, 2012, he bought a Smith & Wesson M&P-15 semi-automatic rifle from a Gander Mountain store in Thornton, Colo. He also bought a shotgun and two Glock semi-automatic handguns. He passed the background checks.

Over the Internet, Holmes purchased two magazine holders for the M&P15 and 3,000 rounds. That is, he modified the assault weapon so it could fire even more rounds without being reloaded. By the way, "M&P" stands for "military and police," which was Smith & Wesson's original intended clientele decades ago. In contrast, the M4 now used by many U.S. troops in war zones was originally envisaged as a lightweight weapon for police SWAT teams. With his modifications, Holmes may have been better armed than U.S. infantrymen in Afghanistan.