There are many things about the Parkland, Florida, high school massacre that are depressingly familiar. One is the weapon used by the 19-year-old suspect.

It was an AR-15. The same lightweight, semi-automatic rifle used in many other mass shootings, including Newtown in 2013, San Bernardino in 2015, and Las Vegas and Sutherland Springs last year.

Local media in Florida report the suspect bought his gun legally. In Florida, there’s no waiting period for would-be buyers of rifles or other long guns.

The National Rifle Association has called the AR-15 the “most popular rifle in America,” and estimates Americans own more than 8 million of them. The NRA says the gun is popular because it’s “customizable, adaptable, reliable and accurate.” Those features may also explain why it’s also become a weapon of choice for mass shootings.

Peter Squires, a specialist in gun violence at the University of Brighton in England, says the way the AR-15 has been marketed by Colt, its manufacturer, has added to its appeal.