Mass killings in the U.S., like Friday's schoolhouse slaughter, have become a troubling and recurring fact of life in America.

In the latest carnage, authorities say a gunman killed at least 26 people, including 18 students, inside an elementary school in the northeastern state of Connecticut.

In July, a troubled graduate student opened fire at a midnight showing of the latest Batman movie at a Colorado theater, killing 12 people. Less than a month later, an Army veteran killed five men and a woman at a Sikh temple in Wisconsin.

Like Friday's assault, the killings have often occurred in seemingly peaceful settings. A gunman in early 2011 killed six people and wounded 13 others, including a U.S. congresswoman, as she was meeting with voters on a Saturday morning outside a grocery store in Arizona.

In 2009, an Army psychiatrist killed 13 soldiers and civilians on an Army base in Texas.

Two years earlier, a student at a large university, Virginia Tech, killed 32 people on the sprawling campus. In 1999, two students at a Colorado high school killed 12 of their classmates and a teacher.

The Mother Jones magazine says that since 1982, there have been at least 61 mass murders in the U.S., which U.S. authorities define as an assault in which a gunman kills four or more people, typically in a single location.

After mass killings in the U.S., some lawmakers have called for much tighter gun controls.

But U.S. officials have only occasionally adopted new laws, because the country's Constitution guarantees the right to bear arms. Mother Jones said that in the mass murders it cataloged over the last 30 years, gunmen used 139 weapons, with more than three-quarters of them obtained legally.