On this date two years ago, the nation was horrified when a gunman killed six and wounded 13 outside a supermarket in Tucson, Ariz. One of those seriously injured was U.S. Rep. Gabrielle Giffords. Those killed included a judge and a 9-year-old girl.

The shooter used a semi-automatic pistol — one in which every pull of the trigger shoots a bullet — with a 33-round magazine.

The outcry from the shooting was immediate. There were calls for more control of semi-automatic guns — including thorough background checks — and for a ban on extended magazines. Americans were horrified and angered. And then they forgot.

America received plenty of reminders on the need to do something to curb mass shootings. On Sept. 6, 2011, a shooter shot 12, five fatally, at an IHOP in Nevada. A month later, a shooting at a hair salon in California killed eight and injured one.

A student killed three students and injured three more at a high school in Ohio in February 2012. In April 2012, a shooter killed seven at a California college. Four days later, five men were shot in Oklahoma. On May 29, 2012, a shooter killed five people at a coffee shop, later killing himself.

Twelve died and 58 were wounded in July 2012 when a shooter went on a rampage at an Aurora, Colo., movie theater. In August 2012, a shooter killed six members of a Sikh temple in Wisconsin, then himself, and injured four. In September 2012, a former employee killed five people at a sign company in Minneapolis, injured three and killed himself.

Then came the tragedy of Dec. 14, 2012, when a gunman killed 20 children at Sandy Hook Elementary in Newtown, Conn., killed six staff members, killed his mother and killed himself. He used a semi-automatic rifle with an extended magazine.

America is not unique in its incidence of mentally ill citizens, although it is worse than most at caring for them. Nor is it unique in the prevalence of violent video games and movies. America is unique, however, in the ready availability of semi-automatic weapons and extended magazines.

The rate of people killed by guns is 20 times higher in the United States than in other developed nations. In the last three decades, America has experienced 61 mass murders.

It is time for Americans to take action. No solution will eliminate all mass shootings, but the experience of other nations makes clear that sensible legislation limiting access to semi-automatic guns and extended magazines is a good place to start.